SOLAR ENERGY CONVERSION

SOLAR ENERGY CONVERSION

1. Introduction and basics of solar energy

The energy sources of the future are being discussed concerning the global climate change caused by the human activities mostly connected with exploitation of fossil fuels (oil, coal, gas). On the other hand the reserves of these fossil energy source also called conventional are going to their end. From this point of view only the renewable energy sources satisfy the ecological requirements and environment safety.

The low of energy conservation states that the energy cannot be created or destroyed, only conversion from one form to another is possible. The energy can be transformed and it is possible to exit in different forms. So the law can be applied to the solar energy conversion. The solar energy can be transformed into heat of a body, only if the body is exposed to sun beams. So the solar energy also is form of energy in accordance with the energy conservation law. The act of conversion is the contact of solar radiation with a body surface.

Simply the contact and transformation of the solar energy may be presented so, fig.1 :

Fig.1. Principle of the thermal use of solar energy (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

There are also other conversion processes - chemical energy into thermal, thermal into mechanical (internal combustion engines), electrical to thermal, etc.etc. In the case of solar energy transformation, the characteristics of the body receiving the solar energy are very important. The body is so called receiver and its properties are determining the quantity of the energy of transformation. In Table 1 some characteristics of the materials when they are in process of interaction with solar radiation are given :

Table 1: Possibilities for using solar energy.

Thermal Radiation

The theory of light as a particle flux is developed by Max Planck and Albert Einstein. This concept differs from the classical wave theory. Their interpretation is based on the assumption that the radiation is a quantised flux. Also Niels Bohr has formulated the complementary principle. It states that microscopic physical systems can only be described by mutually exclusive models. At microscopic level, the structure is different to those at macroscopic. So, the postulated wave packets called photons (in Greek light) are carrying the following characteristics : They are invisible and can be created or annihilated. The photons have no mass. Each photon can transport only a defined amount of energy light quantum. If the photons of the same frequency have same energy, the intensity of radiation is determined by the photons number.In spite of mass lack, the photons are acting some way like material particles. It is accepted to thing of the photons in a way of wave packet, connected with the frequency and energy quantity E = h ., where h is the Plank constant and " is the frequency. After the contact of the photons with the material body, a transfer of energy and momentum is transferred. Every photon has wavelength, connected with its energy. The shorter wavelength of the photon is, the higher energy it has. So the radiation of the Sun is flux of photons with different energy and wavelength. The distribution of photons in dependent of their wavelength is called spectrum. The human eyes can fill only a part of this spectrum, which we accept as visible, fig.2. The photons can also create a visible colour effect in our eyes, fig.3 and the colours correspond to wavelength or energy.

Fig.2. Solar radiation spectrum (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

Fig.3. Colours in the visible range of the solar spectrum (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

Heat

The heat is a form of energy, caused by interactions of the atoms. Even in the solids, the atoms are not still, oscillating around their position. It is accepted the temperature is a measure of kinetic energy of the atoms and the movement of these microscopic particles is in dependence of their temperature. The thermodynamic postulates when heat is supplied to a body, the atomic motion of the bodied increases and its internal energy rises.

Transfer of energy by thermal radiation to a body

The theory says a photon can transfer its energy completely to the atom. After the energy transfer, the photon does not exist anymore. All of the processes of interaction between photons and bodies surface can be explained with 3 simple conditions, fig.4 and consideration the macroscopic body :

Absorption the photon is absorbed by the body.

Reflection the photon is reflected by the surface.

Transmission the photon passes through the body.

The theory is dividing the bodies into 3 parts and also the combinations between them, fig.4 :

Absolute black body all the photons are absorbed. The Sun has the characteristics of an absolute black body

Ideal reflector all the photons are reflected from the surface.

Absolute transparent body all the photons are passing through the body.

Grey bodies part of the photons are absorbed and the rest of them reflected.

Fig. 4. Types of interaction between radiation and matter (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

Simplified, it is possible to make a connection between the brightness and the capabilities of the bodies to receive energy as heat, Table 2.

Table 2: Brightness and heating up of objects.

The absorption properties of the different materials are very important in order to use them to receive the energy from the whole solar spectrum, Table 3.

Table 3: Absorption properties of different materials.

Solar radiation on the Earth's surface

In order to analyse the radiation as a phenomenon, it is very important to know the quantity of the solar energy on the Earths atmosphere boundary and Earths surface. The solar radiation can be treated as intensity of the energy flow, or so called flux energy per 1 m2 surface for 1 second. The dimension is W/m2 or kW/m2 and the solar flux intensity is called irradiance or this is the power of the solar radiation. On the Earths surface the behaviour of the irradiance varies according hours of the day, day and night, seasons and weather conditions. Even within a date of the year, the irradiance can vary because of different weather conditions and from this point of view, the irradiance often has stochastic character. The energy caused by the irradiance is called irradiation. Its dimension is Wh/m2 or kWh/m2 and it represent the integration value over a period of time, Table 4.

Table 4: Units to measure radiation.The extraterrestrial radiation to the Earths atmosphere is in the boundaries 1320 W/m2 1420 W/m2. It is assumed the radiation on the Earths surface to be known as Solar constant and its value is Eo = 1367 W/m2.

After transmission of this radiation through atmosphere, its value is reduce by absorption, reflection, scattering and about 1000 W/m2, max 1200 W/m2 incident on the surface of the Earth at midday when the sky is clear. The components of this global radiation are :

Direct ( beam ) component coming directly from the Sun.

Diffuse component reflected from the bodies around. When the sky is cloudy, only the diffuse component is present, Table 5.

The radiation is function of the latitude and longitude and the Sun position during the day and seasons weather. The largest values of the irradiance are observed at midday during June and July. The winter months are with minimal irradiance, connected with Sun and Earth motion.

Table 5: Radiation intensity for various weather conditions (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).It is considered the diffuse radiation in Central is within the range of 40% (May) to 80% (December) of the global [1]. This fact is very important when building projects based on solar energy exploitation.

Fundamentals of solar energy exploitation

In order to convert as much solar energy as possible with minimal loses the systems must answer some requirements. Especially the requirements are important for the receiver, i.e. the body which has to absorb the solar energy and to transfer it :

The receiver should absorb solar energy from the whole spectrum without loses.For this purpose the absorbers are made with special covering ready to absorb as much as possible energy and to reduce the reflection to minimum.

The quality of conversion may be expressed with the efficiency, Useful energy transferred

= ((((((((((((((

Radiation on receiver surface

There is one more factor for a better performance of the system and it is connected with the energy income. This is the receiver orientation to the Sun. There are two types of systems stationery and tracking. The first is oriented to a fixed position with two angle parameters azimuth and tilt. The azimuth angle in the Northern hemisphere is oriented to South and the tilt angle is from horizontal surface to the Sun. For the different locations optimal tilt angle has been obtained, fig.5. The tracking systems are similar to sunflower. They are single axis (moving to azimuth) and two-axis (moving to azimuth and up to the Sun). They are more efficient, but also more expensive.

Fig.5. Diffuse radiation on a tilted receiver (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).The maximal sola gain can be realized if the surface of the receiver is normally oriented to the radiation. For instance in Central Europe small tilt angle is preferable, fig.6 [1] :

Fig.6. Direct radiation on a tilted receiver (Image: Fraunhofer ISE, Freiburg, Germany; Solarpraxis AG, Berlin, Germany).

The optimal tilt angle is connected with the system requirements, Table 6 :

Table 6: Receiver tilt angle for different conditions (in Central Europe).

Obviously the minimal tilt angle must be on the equator and the maximal on the poles.

The other important questions are about the absorbers area and the energy storage capacity thermal or electrical in standalone systems. Both parameters must be calculated very carefully based on observations and measurement of solar energy parameters due to the local characteristics of the site where energy conversion system is provided. The absorber area and storage capacity are crucial for answering the energy consumption needs during the whole year, day and night.

2. Data collection technical systems for measurement of solar energy parameters

The main parameters of the solar energy are solar irradiance, W/m2, i.e. the solar energy flux intensity, the ambient temperature, oC and duration of solar shining in days. The practical measurement of irradiance is provided by technical devices, most precise are so called pyranometers and different constructions of silicon sensors. In fig.7 pyranometer production of Kipp & Zonen, type CM11 is shown. The CM11 pyranometer is intended for high accuracy total global, or diffuse sky, solar radiation measurement research on a plane/level surface. The CM11 is fully compliant with the ISO-9060 Secondary Standard pyranometer performance category (highest ISO performance criteria for a pyranometer). Extremely high mechanical tolerances are maintained during manufacture to ensure optimal measurement performance in the field [2].

Fig. 7 Kipp & Zonen CM11 pyranometer

Most convenient for complex technical systems for meteorological measuremnents are the silicon sensors, fig 8. The LI-200 Pyranometer is designed for field measurement of global solar radiation in agricultural, meteorological, and solar energy studies. In clear, unobstructed daylight conditions, the LI-COR compares favorably with first class thermopile-type pyranometers, but is priced at a fraction of the cost.

Fig.8. LI-COR 200 silicon sensor for irradiance.

The systems for data collection include different sensor for solar irradiance, temperature, atmospheric pressure and relative humidity, data logger and data transfer system. The data logger registers records and stores the measured data. The row data may be transferred by different technical decisions Radio Frequency transfer (RF), Bluetooth and post popular and efficient for long distances GPRS with SIM cards communicator. This approach is very suitable, because the data may easy received by e-mail. After the data transfer, the data can be treated different software.

3. Methodology for Solar Potential Assessment

The assessment of the solar potential is very important for all projects, concerning exploitation of solar energy. The solar radiation has stochastic behavior. It is not a constant value and depends on the site position, meteorological conditions, local weather conditions and other factors. In order to assess the solar potential on a specific site, the theory has not a precise answer. They are theoretical methods for calculation the different types of radiation, in this number direct and diffuse, but these methods do not calculate the local weather conditions, which are dependent of a lot factors and they cannot be predicted. Something more, during the same seasons, the weather change itself so that on a fixed date of the year the solar irradiance can vary within wide range.

In order to obtain a preliminary value for the yearly solar gain, the solar irradiance or the irradiation have been statistically observed on different sites and solar maps built. Also it is possible to observe via satellite the weather condition and this is also opportunity for a local solar assessment. Unfortunately these methods do not describe correct the solar irradiance on a specific place during the year where a solar energy conversion facility is planned for building and the project cost are in hard dependence with the solar gain.

One of the most popular systems for solar irradiance and irradiation prediction for specific geographical coordinates (latitude and longitude) is so called PVGIS. This is Photovoltaic Geographical Information System which includes a data base for interactive access to the solar potential for different geographical coordinates. The data base includes solar data for a period of 25 years. Based on the solar data, a lot of so called solar maps are charted. The maps represent data long-term yearly averages of selected climatic parameters. The original data (with 1-km grid resolution) have been aggregated into files with grid size of 5 arc-minutes [3]. On the following figures, two solar maps are presented. The first is for the global irradiation on horizontal surface, fig.9, the second for global irradiation on optimally-inclined surface, fig.10.

Fig.9. Global irradiation on horizontal surface, kWh/m2, PVGIS.

Fig.10. Global irradiation on optimally-inclined surface, kWh/m2, PVGIS.

For preliminary assessment the solar potential a PV estimation utility is available [4]. The utility is interactive and by latitude and longitude give, some solar parameters are calculated, fig.11.

Fig.11. Screen of the GIS interactive maps.

For instance, following coordinates are used in order to calculate the monthly solar irradiation : 4424'8" North, 260'56" East.

The resulting window shows the following :

Monthly Solar Irradiation

PVGIS Estimates of long-term monthly averages

Location: 4424'8" North, 260'56" East, Elevation: 83 m a.s.l.,

Solar radiation database used: PVGIS-classic Optimal inclination angle is: 34 degreesAnnual irradiation deficit due to shadowing (horizontal): 0.0 %

Month

Hh

Hopt

H(90)

Iopt

T24hNDD

Jan

1330

2150

2200

63

-1.3

579

Feb

2040

2910

2640

56

1.9

420

Mar

3140

3850

2930

43

6.1

330

Apr

4480

4870

2960

28

11.8

114

May

5640

5580

2790

15

17.7

15

Jun

5990

5660

2560

8

21.5

1

Jul

6100

5910

2770

12

23.6

0

Aug

5530

5860

3260

25

23.0

5

Sep

4260

5220

3730

41

17.5

79

Oct

2890

4160

3690

54

12.3

279

Nov

1500

2310

2270

61

6.3

461

Dec

1100

1780

1840

64

0.2

597

Year3680

4200

2800

34

11.7

2880

Hh: Irradiation on horizontal plane (Wh/m2/day)

Hopt: Irradiation on optimally inclined plane (Wh/m2/day)

H(90): Irradiation on plane at angle: 90deg. (Wh/m2/day)

Iopt: Optimal inclination (deg.)

T24h: 24 hour average of temperature (C)

NDD: Number of heating degree-days (-)

PVGIS European Communities, 2001-2010Reproduction is authorized, provided the source is acknowledged

As seen of PVGIS grid resolution 1 km, the correct data may only be obtained by solar parameters measurement on the very place where energy conversion facility has to be built. Something more, it is necessary to perform the measurement during a period of one calendar year. Then the results may be compared with the statistical GIS data in order to make a precise preliminary comparison between investment and solar gain.

For solar parameters measurement a different sensors and systems for data management and collection are used.

As a result, the following procedure for solar irradiation may be performed :

1. After data collection, the solar irradiance must be integrated to find the irradiation for a given day.

2. After all days of the month irradiation calculation, the monthly irradiation can be obtained.

3. The same way after summarizing the monthly irradiation, the whole energy for the year or the solar potential for the period of measurement is known.

The following example shows the realization of the procedure.

Example

Measurement values for 9-th of June 2008 by the data logger are given. The measurement is performed on a horizontal surface. The data format is as follows :

Time / Irradiance, W/m2 / Time / Ambient temperature, oC

3:30:00 AM

0

3:30:00 AM

14.2

3:37:30 AM

0

3:37:30 AM

14.1

3:45:00 AM

0

3:45:00 AM

13.7

3:52:30 AM

0

3:52:30 AM

13.6

4:00:00 AM

0

4:00:00 AM

13.6

4:07:30 AM

1

4:07:30 AM

13.6

4:15:00 AM

3

4:15:00 AM

13.8

4:22:30 AM

7

4:22:30 AM

13.8

4:30:00 AM

11

4:30:00 AM

14.1

4:37:30 AM

16

4:37:30 AM

14.3

4:45:00 AM

21

4:45:00 AM

14.3

4:52:30 AM

24

4:52:30 AM

14.3

5:00:00 AM

28

5:00:00 AM

14.3

5:07:30 AM

32

5:07:30 AM

14.4

5:15:00 AM

36

5:15:00 AM

14.8

5:22:30 AM

39

5:22:30 AM

14.9

5:30:00 AM

44

5:30:00 AM

15.3

5:37:30 AM

47

5:37:30 AM

15.6

5:45:00 AM

50

5:45:00 AM

15.9

5:52:30 AM

56

5:52:30 AM

16.1

6:00:00 AM

216

6:00:00 AM

16.4

6:07:30 AM

242

6:07:30 AM

16.9

6:15:00 AM

182

6:15:00 AM

17.3

6:22:30 AM

290

6:22:30 AM

17.8

6:30:00 AM

316

6:30:00 AM

18.4

6:37:30 AM

338

6:37:30 AM

19

6:45:00 AM

365

6:45:00 AM

19.6

6:52:30 AM

390

6:52:30 AM

20.3

7:00:00 AM

416

7:00:00 AM

20.9

7:07:30 AM

439

7:07:30 AM

21.6

7:15:00 AM

466

7:15:00 AM

22.4

7:22:30 AM

495

7:22:30 AM

23.1

7:30:00 AM

518

7:30:00 AM

23.7

7:37:30 AM

542

7:37:30 AM

24.2

7:45:00 AM

563

7:45:00 AM

24.6

7:52:30 AM

590

7:52:30 AM

24.9

8:00:00 AM

609

8:00:00 AM

25.4

8:07:30 AM

632

8:07:30 AM

26.1

8:15:00 AM

653

8:15:00 AM

26.6

8:22:30 AM

674

8:22:30 AM

27.3

8:30:00 AM

697

8:30:00 AM

27.6

8:37:30 AM

718

8:37:30 AM

28.4

8:45:00 AM

735

8:45:00 AM

29.3

8:52:30 AM

758

8:52:30 AM

30.1

9:00:00 AM

775

9:00:00 AM

31.1

9:07:30 AM

797

9:07:30 AM

32.4

9:15:00 AM

812

9:15:00 AM

33.9

9:22:30 AM

823

9:22:30 AM

35.3

9:30:00 AM

839

9:30:00 AM

36.5

9:37:30 AM

864

9:37:30 AM

37.5

9:45:00 AM

878

9:45:00 AM

38.4

9:52:30 AM

890

9:52:30 AM

39.1

10:00:00 AM

906

10:00:00 AM

40.7

10:07:30 AM

933

10:07:30 AM

41.5

10:15:00 AM

925

10:15:00 AM

43.8

10:22:30 AM

896

10:22:30 AM

43.8

10:30:00 AM

982

10:30:00 AM

42.8

10:37:30 AM

991

10:37:30 AM

47.1

10:45:00 AM

994

10:45:00 AM

46.1

10:52:30 AM

1027

10:52:30 AM

46.5

11:00:00 AM

1033

11:00:00 AM

43

11:07:30 AM

1055

11:07:30 AM

43.1

11:15:00 AM

997

11:15:00 AM

44.6

11:22:30 AM

994

11:22:30 AM

47.8

11:30:00 AM

1009

11:30:00 AM

43.6

11:37:30 AM

1017

11:37:30 AM

46.9

11:45:00 AM

985

11:45:00 AM

46.9

11:52:30 AM

1032

11:52:30 AM

46.7

12:00:00 PM

1038

12:00:00 PM

47.3

12:07:30 PM

306

12:07:30 PM

50.3

12:15:00 PM

1040

12:15:00 PM

45.8

12:22:30 PM

1039

12:22:30 PM

47.7

12:30:00 PM

1029

12:30:00 PM

44.9

12:37:30 PM

1010

12:37:30 PM

45.4

12:45:00 PM

997

12:45:00 PM

46.5

12:52:30 PM

994

12:52:30 PM

46.9

1:00:00 PM

1002

1:00:00 PM

46.8

1:07:30 PM

1012

1:07:30 PM

51.6

1:15:00 PM

1042

1:15:00 PM

48.9

1:22:30 PM

998

1:22:30 PM

49.6

1:30:00 PM

902

1:30:00 PM

48.1

1:37:30 PM

856

1:37:30 PM

45.3

1:45:00 PM

251

1:45:00 PM

47.9

1:52:30 PM

932

1:52:30 PM

44.8

2:00:00 PM

914

2:00:00 PM

44.9

2:07:30 PM

882

2:07:30 PM

47.9

2:15:00 PM

447

2:15:00 PM

47.6

2:22:30 PM

682

2:22:30 PM

48.4

2:30:00 PM

797

2:30:00 PM

45.8

2:37:30 PM

811

2:37:30 PM

43.8

2:45:00 PM

777

2:45:00 PM

43

2:52:30 PM

775

2:52:30 PM

44.7

3:00:00 PM

751

3:00:00 PM

47.1

3:07:30 PM

727

3:07:30 PM

46

3:15:00 PM

719

3:15:00 PM

44.1

3:22:30 PM

696

3:22:30 PM

43.9

3:30:00 PM

677

3:30:00 PM

45.2

3:37:30 PM

174

3:37:30 PM

45.3

3:45:00 PM

715

3:45:00 PM

37.2

3:52:30 PM

599

3:52:30 PM

34

4:00:00 PM

555

4:00:00 PM

32.4

4:07:30 PM

552

4:07:30 PM

31.3

4:15:00 PM

579

4:15:00 PM

31.3

4:22:30 PM

578

4:22:30 PM

32.2

4:30:00 PM

526

4:30:00 PM

33.9

4:37:30 PM

510

4:37:30 PM

35.3

4:45:00 PM

462

4:45:00 PM

36.6

4:52:30 PM

418

4:52:30 PM

37.9

5:00:00 PM

377

5:00:00 PM

35.3

5:07:30 PM

316

5:07:30 PM

34.8

5:15:00 PM

233

5:15:00 PM

34.1

5:22:30 PM

164

5:22:30 PM

33.4

5:30:00 PM

89

5:30:00 PM

30.6

5:37:30 PM

56

5:37:30 PM

29.3

5:45:00 PM

58

5:45:00 PM

26.4

5:52:30 PM

70

5:52:30 PM

24.8

6:00:00 PM

72

6:00:00 PM

24.2

6:07:30 PM

72

6:07:30 PM

23.9

6:15:00 PM

66

6:15:00 PM

23.7

6:22:30 PM

57

6:22:30 PM

23.4

6:30:00 PM

45

6:30:00 PM

23.4

6:37:30 PM

34

6:37:30 PM

23.2

6:45:00 PM

23

6:45:00 PM

22.8

6:52:30 PM

19

6:52:30 PM

22.5

7:00:00 PM

14

7:00:00 PM

22.2

7:07:30 PM

10

7:07:30 PM

22

7:15:00 PM

4

7:15:00 PM

21.8

7:22:30 PM

2

7:22:30 PM

21.6

7:30:00 PM

1

7:30:00 PM

21.4

7:37:30 PM

0

7:37:30 PM

21.3

7:45:00 PM

0

7:45:00 PM

21.3

7:52:30 PM

0

7:52:30 PM

21.1

8:00:00 PM

0

8:00:00 PM

20.9

8:07:30 PM

0

8:07:30 PM

20.8

8:15:00 PM

0

8:15:00 PM

20.7

8:22:30 PM

0

8:22:30 PM

20.6

8:30:00 PM

0

8:30:00 PM

20.4

The data from the logger must be time integrated in order to find the irradiation from the irradiance. The time interval of the logger is 7 minutes and 30 seconds, i.e. 7,5 minutes or 7,5/60 hours.

The irradiation can be expressed as follows :

( 1 ) ,

where

E is irradiation, Wh/m2.

I is irradiance, W/m2.

to, tn are the starting and final time of integration, hours.

( 2 ) ,

where

k is the 1-st measured irradiance value, n the last one.

time step, hours, i.e. data logger time interval.

The precision of ( 2 ) is good enough, because if the rule of trapezoids is implemented, the only error will be at the beginning and at the end of integration. As seen all the values in the midnight are zero, so they will not affect the integral value.

Finally, the sum of irradiance data is 66266 W/m2 and the time interval is 7,5 / 60 hours = 0,125 hours. The irradiation is equal to 8283 Wh/m2 for the 9-th of June.

In the figures below, the measured data for irradiance and temperature are presented.

Solar Irradiance, W/m2

Fig.12. Measured irradiance, W/m2 for 9-th of June.

Ambient temperature, oC

Fig. 13. Measured ambient temperature, oC for 9-th of June.

For comparison, the irradiance and ambient temperature for 14-th of February are given. June and July are the months with highest irradiance and the winter months with lowest.

Solar Irradiance, W/m2

Fig.14. Measured irradiance, W/m2 for 14-th of February.

Ambient temperature, oC

Fig.15. Measured ambient temperature, oC for 14-th of February.

The irradiation for 14-th of February is 1681 Wh/m2. Measuring ambient temperature is very important, because of days of freezing and the days with high temperature. In both cases the efficiency of the solar cells drops down, freezing lowers the optical characteristics and high temperature lowers the cells efficiency.

After the monthly irradiation data has been calculated, the sum gives the potential, fig.16.

Solar irradiation, Wh/m2

Fig.16. Total potential for the year : 1330 kWh/m24. Basics of solar thermal conversion

These systems for solar thermal conversion collect and store solar energy as heat. They produce heated water for domestic consumption and heating. The main elements of the solar thermal systems for water heating include collectors, storage tanks and plumbing systems. Other type of solar thermal systems produce directly heat air for space heating, [5]. Other area of implementation of solar energy is solar refrigeration based on adsorption and absorption processes, but still not widely developed due to the high price of the equipment.

Solar technologies are widely spread for the last 30 years, but known for a thousand of years. The applications of solar thermal conversion may de described generally as [6] :

Solar water heating (thermosyphon, integrated collector storage systems, air systems, direct circulation and indirect water heating systems).

Solar space heating systems (water and air systems).

Solar refrigeration (adsorption and absorption systems).

Industrial process heat systems (low temperature air and water based, steam generation systems).

Solar desalination (direct solar stills, indirect conventional desalination).

Crop drying.

Solar thermal power generation systems (parabolic trough systems, the power tower or central receiver systems, parabolic dish systems, Stirling engine).A classification of the applications is proposed in [7]. This classification is performed by the temperature range of the systems :

Low temperature.

Medium temperature.

High temperature.A large amount of the solar thermal facilities are working within the temperature interval < 43 oC. The heat transfer medium is liquid or air and these systems are used for domestic needs. The medium interval includes range 60-82 oC. The working media is also liquid and air. They also can be used for domestic needs and small industrial equipment. The high interval is for temperatures more than 82 oC. In order to obtain high temperatures, the facilities are concentrators like mirrors or lenses. The media is transferring heat to other media (most often water) in order to create superheated steam for electricity and heat production. In order to perform 24 hours operation, the systems use heat exchanging elements to store heat.

Another classification gives collectors distribution based on their performance. They can be :

Flat solar collectors for liquid media or air. Can be glazed or unglazed.

Evacuated tube solar thermal collectors with direct flow or heat pipes.

Concentrating collectors with parabolic concentrators, power towers or solar cookers.

5. Technologies development of solar thermal installationsThe flat solar collectors for liquid media are usually glazed for better performance. They are cheaper than evacuated and have good cost performance ratio. By the German Solar Society (2005) a glazed flat plate collector can collect approximately 450 kWh/m2 per year.

Flat collectors, fig.17 consist of the following basic elements :

Casing with insulation of the back side.

Glass Cover.

Absorber plates with pipes.

Fig.17. Flat plate solar collector principle scheme.

The solar radiation passes through the Glass Cover and is heating the absorber plates with the pipes. Then by heat conduction in absorber and pipes and following convection the heat is given to the fluid in pipes causing rising of its temperature. In order to reduce convection heat loses, on the back side of the casing thermal insulation is provided.

The next step of the solar thermal technologies is implementation of evacuated tubes with direct fluid flow or for better performance with heat pipes. The construction of these collectors uses better evacuation between the Glass Tube, fig.18 and the inner tube with the fluid inside. The evacuation is provided in order to decrease the heat losses from the inner tube. For a better performance also a Reflector is provided causing better collection of the solar radiation to the inner tube placed in the focal point of the Reflector.

Fig.18. Heat pipe evacuated tube collector

The typical positioning of the solar collectors is roof incorporated, fig.19 or standalone construction, fig.20.

Fig.19. Roof incorporated flat solar thermal collector

Fig.20. Thermosyphon system with water Tank (Photo courtesy of SUNSYSTEM).

The thermosyphon shown in fig.20 needs proper mechanical system for mounting. The performance is better due to the possibility of mounting the surface of the absorber with the optimal angle for the latitude of the geographical site, fig.21.

Fig.21. Mechanical system for optimal angle performance.

The solar water domestic heating systems cannot use directly water flowing through the pipes of the flat solar collectors. The reason is freezing during cold periods which may cause damage of the system. For this purpose some thermal agents like Propylene Glycol and others in so called Active Indirect Systems is used, fig.22.

Fig.22. Active Indirect Solar Thermal System

In order to prevent damage or incident during systems exploitation special precautions have to be taken. The reason is the vessels under pressure. On the other hand electronic equipment also is needed in order to control optimally the systems operation.

In fig.23 an example of such system is given. The elements for a normal work of the system are :

Air vents for prevention air in the pipeline.

Pressure Relief Valve for prevention system of exploding due to high pressure.

Expansion Tank for regulation the pressure of the system. The thermal fluid expands with the rise of temperature and in a closed system the pressure also rises.

Sensors for control and optimal operation.

Controller which operates the circulation pump when the solar irradiance is not sufficient to heat the thermal agent.

Fig.23. Active Indirect Solar Thermal System with equipment for control and operation. (Picture courtesy of U.S. Department of Energy. Energy Efficiency and Renewable energy).6. Task, function and characteristics of the solar thermal installations

Main task to transform the solar energy into heat.

Function as an exchanger and thermal storage.

Characteristics :

Collectors with air flow.

Collectors with liquid flow.

Glazed collectors.

Unglazed collectors.

Evacuated with direct flow.

Evacuated with heat pipes.

Collectors with concentrators :

parabolic through;

compound parabolic;

dishes ( parabolic most );

power towers;

Stationary collectors.

Tracking systems :

single axis,

two axis.

7. Function and requirements of solar storage units

Storage units serve the purpose of a battery that separates the heat input of solar radiation from the user's energy consumption. Since the temporal pattern of energy input usually does not match that of energy consumption, this separation is necessary in all but a few solar-thermal systems. The time period of storage varies between a few hours, days or (in case of seasonal storage), months and strongly depends on the consumption side of the system (e.g. swimming pool, potable water or room heating) and the desired solar fraction [8].

The goal is to store the available solar energy as completely as possible during periods of low power demand and later supply this energy as efficiently as possible when needed.

Requirements on the solar storage unit:

high heat capacity of the storage medium,

good heat efficiency (small tank surface and good insulation),

a thermally well stratified structure of the tank filling,

a desired life cycle of 25 years (or more) for the complete system,

low cost and good availability,

tank and storage medium must be compatible with the environment and hygiene (e.g. potable water)

the system must withstand the expected range of pressures and temperatures.

Due to its good heat capacity, environmental-friendliness and availability, ordinary water is the commonly chosen storage medium. It is possible for domestic water systems to use potable water directly as the storage medium, or, if an auxiliary solar buffer and heat exchanger is used, non potable heating water.

Alternative systems (latent-heat concepts and chemical storage) have entered experimental development and will not be described further at this point. Non of the surveyed systems used storage media other than water.

Types of Storage Units construction :

Portable water storage.

Zink galvanized Storage Tanks

Enamel-Coated Tanks.

Stainless Steel Tanks.

Solar Buffer Tanks.

Lightly Pressurized Buffer Tanks.

Combi Storage Units.

Open-System Buffers.

Seasonal Storage.

Storage Tank insulation

The heat losses of conventional storage tanks are very significant. In order to keep heat losses of solar storage units within reasonable limits, special requirements must be met for insulation, fig.24 :

a small surface to volume ratio of the tank,

closely attached insulation,

complete insulation of the tank, including upper and lower sides,

insulation of all pipe connections and fitted/ accessories,

pipework guided into the storage tank or below the insulation,

avoidance of in-tube circulation (heat losses caused by convection within the pipe).

The size of the solar storage tank depends, of course, on the size of the collector array, and also on the daily hot water consumption during the week (e.g. approximately constant consumption on all week days, or only consumption on work days, e.g. in factories, workshops etc.).

Good insulation of outlets

Poor insulation of outlets

Fig.24. Storage Tank insulation

The aim is for the solar storage tank to store, on an intermediate basis and until needed, the solar heat collected during the day and not directly passed to the consumer. Simulation calculations with variation of the solar storage tank volume were carried out for location in Cologne, Germany, for a solar installation with a hot water consumption in the building of 7m3/d. The calculations were carried out for two types of building, for a residential building with approximately constant consumption on all week days and for a workshop with no consumption on Saturdays, Sundays and public holidays. In addition, the system load for both buildings was changed (70l/(d.m2) and 40 l/(d.m2)) by increasing the collector area from 100 m2 to 170 m2. The calculations presumed that the storage tank volume can be accommodated in one single container. In practice it is often necessary to install two or three storage tanks. This would lead to a slight increase in storage losses, so that the degree of utilization of the system would be slightly less than the values calculated. The results of these simulation calculations are shown in Fig.25.

Consumption profile apartment house, specific load 70 l/(m2.d)

Consumption profile apartment house, specific load 40 l/(m2.d)

Consumption profile workshop no weekend demand, specific load 70 l/(m2.d) on working day

Consumption profile workshop no weekend demand, specific load 40 l/(m2.d) on working day

10 20 30 40 50 60 70 80 90 100

Specific buffer volume [l/m2 collector area]

Fig.25. Degree of utilization of the solar installation as a function of specific storage tank volume (volume per square meter of collector area), for two buildings used differently and for two specific loads respectively (medium quality solar installation components). (Courtesy of [8])8. Power systems with solar thermal collectors

Solar power plants are built in places, where constant and high solar potential is observed. The largest plants are in California, Nevada, Spain, France, China, India and others. Usually they are based on Rankine cycle and it can be simplified on the following fig.26 :

Fig.26. Chart of Rankine cycle with preheated water vapor

The following pictures show different projects with the characteristics mentioned above :

Fig.27. Dish concentrator, multisegmented

Fig.28. PS10, Sevilla, Spain, 11 MW, 624 Heliostat mirrors, 115 m height of the tower

. Fig.29. Solar Two, California. ( Photo courtesy of NREL ).

Fig.30. Evacuated tube collector field in Tibet, China.

Fig.31. Mojave Desert, California, 354 MW, 936384 mirrors, Area 6,5 km2

Fig.32. Nevada Solar One, Las Vegas, 64 MW, 180000 mirrors, 1,6 km29. Management and exploitation of Solar Thermal Systems9.1 Flat Solar Collectors Efficiency calculation

The heat transfer analysis is used in order to assume the efficiency of a flat solar collector [6], [9]. Steady state balance expressed for the incomes Qin and loses Qloses :

, W

(1)

The component comes from the solar radiation

, W

(2)

where

is the transmittance of the glass cover,

absorptance capability of the absorbers plate,

I Solar irradiance, W/m2.

absorbers plate area, m2.

The heat loses can be expressed by :

, W

(3)

where

U is heat transfer coefficient, W/(m2.K), temperature of the plate, oC, ambient temperature, oC.The heat power calculated for the collector fluid :

, W

(4)

where

G mass flow rate of the collectors fluid, kg/s,

Cp specific heat of the fluid, J/(kg.K),

outlet temperature of the fluid, oC,

inlet temperature of the fluid in collector, oC.

The efficiency can be calculated by :

, or

.

(5)

Example 1

A flat solar collector has the following parameters : Area 2 m2, tramsittance-absorptane product : () = 0,82, Heat transfer coefficient 6,5 W/(m2.K). The global solar irradiance to collectors surface is 720 W/m2. The ambient temperature and the absorbers plate temperature and respectively 28 oC and 49 oC. Calculate collector efficiency.

Solution

From (5)

,0,63.

( The difference between Kelvin and Celsius degree has same value. )

9.2 Air Solar Collectors

Air as a fluid is not suitable for heat exchange. The reasons are the low heat capacity and viscosity. On the other hand most of the modern air condition facilities are heating or cooling air for accommodation needs. The realized idea is to heat air by sun radiation and supply it directly. They are two directions of solar air collectors : building integrated and standalone collectors like these heating liquids. The standalone collectors have almost the same construction glazing, frames with insulation and absorber. In this case there is outside air driven by a fan, heating the air in collector and supplying the heated air in accommodation. In fig.33 a standalone air collector is shown. The test parameters of the collectors are :

Active area 1,25 m2.

Orientation South.

Time of test November, 10:30 AM.

Tilt angle to the Horizon 60 deg.

Air mass rate 76,6 kg/hour.

Inlet air temperature 16 oC.

Outlet air temperature 56 oC.

Global Solar irradiance 800 W/m2.

Heat Power of the collector 676 W/m2.

The advantages of this type air collectors are :

Low weight. There are not pipelines under pressure.

No freezing or boiling processes of the air.

Lack of corrosion materials.

Low price of the collector.

Fig.33. Prototype of a standalone air solar collector

Example 2

Air solar collector has mass flow rate of 70 kg/hour. The air heat capacity is 1,0 kJ/(kg.K). The inlet temperature of the air is 5 oC. On the outlet is 34 oC. Calculate the heat power of collector.

Solution

From (4) - ,

G = 70, kg/hour = 70/3600, kg/s = 0,0194 kg/s.

Cp = 1,0 kJ/(kg.K) = 1000 J/(kg.K).

Q = 0,0194 . 1000 . ( 34 5 ) = 564 W.9.3 Practical example of Solar Thermal System with SCHOTT Evacuated Tube Collectors ETC 16. Construction of SCHOTT Evacuated Tubes, fig.34 :

Fig.34. SCHOTT Evacuated Tube ETC 16 with casing, [10]

Fig.35. SCHOTT Evacuated Tube ETC 16, opened, [10]

Parameters of SCHOTT Evacuated Tube ETC 16 :

Scheme of the project :

Fig.37. Solar thermal collector and thermal system

Legend :

A0 automatic air vent;

circulation pump with electronic temperature control;

Expansion Tank with safety valve, 50 l;

water heater (2 serpentine type), 500 l;

automatic control unit. 1 > 2 pump working temperatures; - water steam safety valve, 3 bar; thermometer, manometer; water filter; one-way valve; closing valve; Simulation of the system by Polysun 3.3. Results.

By Polysun 3.3 the system has been simulated. The detailed and graphical reports are presented as follows :

10. 11. Physical basics of solar electricity conversion

Solar cells convert light into electric energy. Very general, this is a three-step process which can be explained by means of fig.36 :

Fig.36. Cross section of a crystalline silicon solar cell (Image: Solarpraxis AG, Berlin, Germany).

Absorption of light, delivering electrons in an excited state. Locally separation of positive and negative charges. Conduction of the charges to an external circuit.Absorption: In most cases the absorber is a semiconductor, and the used transitions are interband transitions. The excited state comprises an electron in the conduction band and a hole in the valence band. These interband transitions are characterised by a sharp long-wavelength on-set of the absorption ("band-edge") and a relatively broad absorption band. So-called "direct" semiconductors have a very high absorption coefficient in their absorption band. In this wavelength regime light is absorbed within a few m of penetration depth. These semiconductors are therefore well suited for thin film solar cells, with a thin semiconductor layer on a low-cost substrate. "Indirect" semiconductors, like the common crystalline silicon, have a much lower absorption, and about 150 m of material are needed to absorb most of the light. As a consequence, relatively thick (200 to 300 m) sheets of silicon so-called "wafers" are the main material for the production of solar cells. Other absorption mechanisms are also used for photovoltaic devices. One well-known example is the well defined absorption of light in organic dyes leading to excited molecular states of the compounds.

Charge separation: Electrons and holes in semiconductors are separated by diffusion or by drift of the charge carriers in the space charge regime of a p-n-junction or in a hetero junction of two materials. Other separation mechanisms are also known, e.g. the tunnelling of electrons through very thin insulating layers. In the already mentioned dye cell the charge separation comprises a charge transfer of the excited electron to the conduction band of a wide band-gap semiconductor, e.g. TiO2.

Conduction of the charges: The charge separation leads to the generation of a voltage between the both sides of a solar cell. Contacts have to be applied to conduct the charges to an external circuit. This is not always easy since the contacts should have a very low contact resistance to avoid electric losses within the device. For semiconductors the right selection of contact materials, combined with the proper technology, may lead to "ohmic" contacts with very good conductance. It is obvious that at least one of the contacts must have a high optical transmittance to allow light to reach the absorber inside the device. One means is to use thin conducting oxides (TCO) as transparent contacts, the other technique is to use small metal fingers ("grid") on top of the cell, tolerating some shadowing (4 to 7 %).

Optimisation of the electric output of a solar cell requires a good coupling of the light to the absorber layer, a high absorption within this layer, and a low recombination of the generated carriers before they are properly separated. The common means to reach this goal are the application of antireflective coatings (ACR) or surface texturing, the use of high purity semiconductors, and the passivation of the surface of the semiconductor. As a consequence, solar cells of acceptable efficiency require elaborate technologies. The development of materials, of the design of the cells, and of the production technologies is in a steady process of optimisation, and this process is far from being completed.

The Solar Cell as a Power Generator

Since the solar cell is intended to deliver the highest possible amount of electric power (at a certain level of incident light) it is useful to analyse the I-V-characteristic with respect to the power which can be extracted. If the operating point of a solar cell is shifted along the I-V characteristic curve (by changing an external load resistor) the electric power generated in the external load can be found by multiplying the current by the voltage in the operating point. This has been done in the graph in fig.37. The power that can be extracted goes through a maximum at a certain point, the Maximum Power Point (MPP). It is characterised by PMPP, the maximum power, by IMPP, the current at maximum power, and by VMPP, the voltage at maximum power.

Fig.37. Power generated by a solar cell as a function of the operating point (Image: Solarpraxis AG, Berlin, Germany).

Fig.38. Dependence of VOC and ISC on the light intensity (Image: Solarpraxis AG, Berlin, Germany).

All electrical parameters of a solar cell are depending on the intensity and the spectrum of the light as well as the temperature of the solar cell. The dependence of current and voltage on the illumination level are shown in fig.38. Whereas the current of the cell is linearly dependent on the irradiance, the voltage and the MPP are not, and therefore the description of the behaviour of a cell under different illumination level is complicated.

For different levels of irradiance but constant cell temperature, this leads to a set of characteristic curves as shown in fig.39.

Fig.39. I-V-curves of crystalline silicon module at different irradiance levels and constant temperature (Image: Solarpraxis AG, Berlin, Germany).

12. Solar Cells, Modules and Photovoltaic Systems

Depending on the technology used, a single solar cell generates a MPP voltage of approx. 0.5 to 2 V. Hence, electrical loads can rarely be run directly at this low voltage unless they are small devices or toys. In general, a higher voltage is necessary. It can be provided by arranging multiple cells in series, as is done with batteries. For instance, 36 crystalline silicon cells are connected in series in standard modules, producing a MPP voltage of approx. 18 V, which is suitable to charge 12 V lead acid batteries. In the meantime, there are standard modules with 72 or more cells, and special modules consist of up to several 100 cells connected in series. In turn, several such solar modules can be connected in series - a string - to form a solar generator that produces voltages up to some hundred Volts.

To ensure the desired solar generator output power, several modules or strings can be connected in parallel, thus increasing the current. This modular interconnection allows photovoltaic generators to be designed with outputs from milliwatts to megawatts - all with the same basic technology.

In the following, the properties of cells and modules connected in series and in parallel will be discussed, particularly their response to partial shading. Then, some typical structures of solar modules will be illustrated and items like lifetime and recycling are presented.

Solar cells and solar modules are connected in series to produce greater overall voltages.

In a series connection, the current is the same in all of the cells so that the overall voltage - as shown in fig.40 for three similar cells - is the result of the sum of the individual voltages.

Fig.40. Series connection of three similar solar cells and their current-voltage (I-V) diagram (Image: Solarpraxis AG, Berlin, Germany).

Connecting solar cells and modules in series does, however, have one major drawback: the weakest link determines the performance of the whole array. Even if only one cell is partially shaded, this cell determines the overall current, and hence the output of the whole string.

Partial shading thus has to be avoided to the extent possible !

Even small shaded areas such as from poles, cables, tree tops, leaves, bird excrement, and dust lead to large output losses and are usually the cause of unsatisfactory energy yield of photovoltaic systems.

The same is true of series connections of cells with different characteristics due to production tolerances or if parts of cells within a module have broken off and hence become inactive. Here, too, the weakest cell determines the overall performance. Cells and modules should be selected and put together so that they produce nearly the same MPP current. This additional work pays for itself when the energy yield increases!

If the system has to produce greater currents, the modules or strings can be connected in parallel as shown in fig.41. In parallel connections, all of the cells have the same voltage, and the overall current is the result of all of the individual currents.

Fig.41. Parallel connection of three similar solar cells and their current-voltage diagram (Image: Solarpraxis AG, Berlin, Germany).As with a series connection, the question is how the shading of a single cell or module will affect performance. In general, the effects on the energy yield are somewhat lower than when the same number of solar cells are connected in series. In particular, there is not usually a danger that the shaded cells will be overloaded by reverse current from other cells in typical arrays. There is thus no need to use string diodes with strings connected in parallel according to IEC 60364 if the modules used are protective class II and their no-load voltage deviates by less than 5 % from each other - conditions that solar power systems generally fulfill. In low-voltage applications such as solar home systems etc., string diodes can generally be omitted.

The wiring diagram and the resulting current-voltage characteristic curves for a solar generator with several solar modules connected in series and in parallel are shown in fig.42.

Fig.42. Modules connected in series and in parallel and their current-voltage characteristics (Image: Solarpraxis AG, Berlin, Germany).13. Electrical systems with photovoltaic power

A photovoltaic system to power an appliance or small consumer generally consists of a photovoltaic generator, a charge regulator, a storage battery, and voltage regulator, fig.43.

In very small systems such as those in watches and pocket calculators, the photovoltaic generator may only consist of one or a few solar cells. To provide greater output, the individual solar cells are wired in solar modules.

Fig.43. Principal design of a photovoltaic system to power an appliance or small consumer (Image: Fraunhofer ISE, Freiburg, Germany).In addition to small modules to power specific products and applications, standard modules are manufactured to provide greater energy supply. These standard modules are dimensioned so that they have a nominal voltage of around 15 to 17 V are thus able to charge batteries with nominal voltages of 12 V.

When selecting the modules, the modules output and voltage have to be correctly dimensioned. Furthermore, the module has to be mechanically constructed to withstand the weather and climatic conditions in the long run. Depending on the available space and the kind of integration, the geometric dimensions and the physical properties and attachment options of the modules frame may also play a role.

To prevent the storage batteries used from being overcharged or deep discharged, a charge regulator is used between the photovoltaic generator, the battery, and the load. The charge regulator generally also contains a discharge protection diode that prevents the battery from discharging over night through the photovoltaic generator. A good charge regulator consumes very little power and has a low voltage disconnect that protects the storage battery from being deep discharged.

The storage battery stores the energy produced by the photovoltaic generator and makes it available to the consumer during bad weather or at night.

Appliances that get their power from photovoltaics mostly use nickel-cadmium batteries (NiCd) or nickel-metal-hydride batteries for storage. However, lead batteries, lithium-ion batteries, and capacitors (called double-layer capacitors) are also used.

In small systems powered by photovoltaics, lead batteries are usually used. Thus, special models of car batteries with extra thick lead plates (called solar batteries) are used for mobile applications, for instance to power electric consumers in campers, boats, and weekend homes. In photovoltaic systems to power homes with permanent residents and daily charging/discharging cycles, usually tubular plate (OPzS") batteries are used. They have deep cycles and hence long service lives. Sometimes, normal car batteries are used in Solar Home Systems because they are more readily available and cheaper.

For some applications, maintenance-free lead batteries are useful; their electrolyte is captured in a fleece or gel. These batteries have 100 times less sulfuric acid vapors than lead batteries with fluid electrolyte, which allows them to be installed in the same housing or space as the electronics. Maintenance-free batteries do not leak and can thus be run in any position.

A voltage regulator may be required to adapt the voltage of the photovoltaic system to the voltage of the consumer. For devices powered by photovoltaics, this regulator is usually a DC/DC transformer, which transforms one direct current into another.

Fig.44. Principal design of a photovoltaic system without energy storage (Image: Fraunhofer ISE, Freiburg, Germany).When demand and supply occur at the same time for systems to power appliances and small consumers, the storage battery is not needed, fig.44. Some examples of these consumers are pocket calculators, fans, and water pumps.

Photovoltaic systems to supply off-grid consumers far from the grid with medium and large output

If greater output levels are required or if conventional household appliances or industrial devices are to be used, the system voltage should be 230 V~. To attain this voltage, an inverter is added to the system, fig.45. The inverter transforms the direct current produced by the photovoltaic generator or the storage battery into alternating current.

Fig.45. Principal design of a photovoltaic system with alternating current output (Image: Fraunhofer ISE, Freiburg, Germany).At our latitude, an energy supply based exclusively on photovoltaics requires large photovoltaic generators due to the fluctuations in solar radiation. The same is true of photovoltaic systems that have to have great availability. Hence, a mixture of generator types is generally combined to form hybrid systems. Combining photovoltaic generators and motor generators ensures the same power security as in the public grid, fig.46.

Fig.46. Principal design of a hybrid system consisting of a photovoltaic generator and motor generator (Image: Fraunhofer ISE, Freiburg, Germany).If solar radiation is good, the photovoltaic generator can meet the whole energy demand in an environmentally friendly way, with no emissions and no noise. Excess energy is stored in batteries. At night or during bad weather, batteries cover the energy demand. When the battery is in danger of being deep discharged, a motor generator such as a diesel or liquified petroleum gas motor is started to cover power needs and recharge the storage batteries at the same time.

Fig.47. Principal design of a hybrid system consisting of a photovoltaic generator, wind turbine, and motor generator (Image: Fraunhofer ISE, Freiburg, Germany).

In windy areas, a wind turbine can be added to the system, fig.47.

As the photovoltaic generator and the wind turbine compliment each other very well when the design is correct, the motor generators operating hours are reduced, thus also lowering the consumption of fossil energy.

In the future, fuel cells will also be used in PV hybrid systems. If users want to be completely independent of fuel supplies and have a wholly autonomous system, an electrolyser and a hydrogen storage system are integrated, fig.48. The electrolyser is operated in the summer when the photovoltaic generator produces excess energy and the lead batteries are fully charged. The hydrogen created is stored and is available for the fuel cell in the winter.

Fig.48. Autonomous power supply from a combination of solar generator and yearlong hydrogen storage system (Image: Fraunhofer ISE, Freiburg, Germany).Grid-connected photovoltaic systems

Photovoltaic systems can feed power to the public grid via an inverter. The advantage is that no energy storage is required as the energy can be used elsewhere, i.e. the output of conventional power plants is reduced. The share of solar energy fed to the grid is currently small, but future energy scenarios forecast an important role for solar energy. State subsidy campaigns (such as the 100,000 Roof Program in Germany), rates for the power fed to the grid high enough to cover costs, and legal frameworks (such as the Renewable Energy Act in Germany) promote the release of such systems on the market.

If compensation for the electricity fed to the grid from small, distributed systems is lower than the price for the electricity from the public grid, the inverter feeds the power directly into the houses lines, fig.49.

Fig.49. Small, distributed photovoltaic system that feeds power into the houses lines (Image: Fraunhofer ISE, Freiburg, Germany).On sunny days, it provides power to the consumers in the house, while excess power is fed to the grid and metered. If the weather is bad and at night, the house gets its power from the conventional grid. The output of such systems is approx. several kWp.

If the compensation for the solar power fed to the grid is greater than the purchase price for electricity from the grid, the system is generally designed as shown in fig.50.

Fig.50. Small, distributed photovoltaic system that feeds power directly to the public grid (Image: Fraunhofer ISE, Freiburg, Germany).The modular character of photovoltaics also allows for large, grid-connected power plants to be built. They feed power directly into the medium or high voltage grid via an inverter.

Fig.51. Principal design of a grid-connected photovoltaic power plant (Image: Fraunhofer ISE, Freiburg, Germany).

Photovoltaics in distributed grids

The deregulation of the European energy market means that the energy sector is becoming more competitive for the generation, distribution, and trading of electricity. New actors will enter the market thanks to the open access to power grids and appropriate conditions for third-party use of such grids. They will strengthen the current trend toward the distributed generation of electricity. In the process, photovoltaic systems can become part of new supply structures. Fig.52 shows an example of what the distribution structure could look like in the future with distributed electricity generation systems.

Fig.52. Principal design of a future energy supply system with distributed electricity generation (Image: Fraunhofer ISE, Freiburg, Germany).14. Computer design and simulation of photovoltaic systems

User requirements for software tools can vary considerably. In the case of many standard system configurations it is sufficient to look only at the system energy flow, or perhaps to just have design assistance for the coordination of individual components. Many inverter manufacturers offer tools that specifically address these needs. These tools largely prevent some of the more primitive dimensioning errors during the coordination of PV modules and inverters in grid-connected PV systems. On the other hand, there are many cases in which detailed time-step calculations over the course of at least an entire year are absolutely necessary. In the case of grid-connected PV systems, these situations often arise as a result of partial shadowing of the generator surface. With stand-alone systems, for example, is often a question of the security of supply during the winter; with hybrid systems, it is usually a question of economic optimization or the technical fine-tuning of components. According to these different requirements, the software can be divided into categories.

Dimensioning tools: This category includes, for example, utility programs from inverter manufacturers. These programs allow users to tune the PV generator to a chosen inverter. The programs base their calculations on predefined maximum values for the generators operating temperature and the resulting electrical maximum values for the voltage levels. Using such programs should eliminate the occurrence of serious error messages in grid-connected PV systems. These tools can generally be downloaded for free from the websites of the leading inverter manufacturers (for example: SMA, Fronius, Mastervolt, and Siemens).

Programs for system simulation: Solar power system simulations, in other words the reproduction of PV systems with the help of a computer, require good statistical and dynamic descriptions of the individual components, which need to be accurately linked to the complete system. Moreover, detailed system simulations require calculations in small, temporal increments. For energy yield calculations of PV systems, an hourly basis is an established. In the meantime, almost all of the simulation programs on the market are included in this group.

Open simulation environments: Yet users who frequently conduct system simulations using the above tools quickly come to recognize their limitations for grid-connected PV systems, naturally, less than with hybrid systems. With hybrid systems, users often find that the pre-defined framework supplied by the program does not allow them to simulate certain configurations. This is especially true when connecting multivalent systems for instance, the connection of building-integrated PV systems at the level of electrical energy, thermal requirements, and daylight use. Whereas programs like InselDi or Modes can tackle particular problems with multivalent systems, it is often more flexible to use open simulation environments like MATLAB / SIMULINK. With these programs, users can implement almost everything themselves. In fact, they have to, since these programs have very limited direct access to block descriptions or libraries.

Databases and utility programs: Although databases and utility programs do not directly fall under the category of system simulation software, some of them play an important role in detailed calculations. The most prominent example in this category is the climatologic database Meteonorm an important tool for the field of irradiance data.

Evaluating the Simulation Results

For grid-connected PV systems, a bad system design quickly results in a poor economic performance. For remote, off-grid systems, a bad design can lead to either system breakdown or to rapid battery ageing. By using simulation programs, such mistakes should be avoided.

In any case, the user must be able to use the program correctly as well as to be aware of potential sources of error. Input errors and the associated erroneous calculations are not uncommon, even for experienced users, with programs having complicated input interfaces.

Several programs, like PV*Sol and PVS 2001, perform a plausibility check of the most important input parameters. Even with this check, however, all errors can not be excluded with certainty. This is why system planners should always validate the simulation results with values gained from experience. For grid-connected systems, the Performance Ratio (PR) or the annual energy output divided by the peak power in kWh/kWp are very good references. These parameters are employed for comparison by most programs.

In Germany the Performance Ratio of the PV system should be greater than 70%. The annual energy yield should be at least 700 kWh/kWp. When the tilt angle of the roof is unfavourable or when shading occurs, the annual energy yield can be less. If simulation results are more than 1000kWh/kWp, or clearly less than 500kWh/kWp for a facade integration or less than 700kWh/kWp for a standard roof integration, then it can be assumed that the input parameters are wrong.

Checking the validity of the simulation results for autonomous systems is, however, more difficult. System planners can check the results using rules of thumb or by comparing them to values gained from experience from PV systems which already exist.

15. Applications of photovoltaic systems

Most of the grid-connected photovoltaic systems in Europe belonging to private system operators are installed as small systems on the roofs or integrated into the roofs of existing houses. Mounting or integrating photovoltaic systems onto or into house roofs or walls is an attractive solution, as double usage is made of the building envelope, and the additional costs for support structures are less (example in fig.53).

Fig.53. Small grid-connected, roof-mounted photovoltaic system (Photo: ZSW, Germany).

Fig.54 shows the schematic structure of a small, grid-connected photovoltaic system. The cables from the individual photovoltaic modules all lead to the generator connection box, which can also contain appropriate protection diodes and surge protective devices. If one inverter is installed for each string of photovoltaic modules, the generator connection box is not required. The inverter converts the direct current (DC) from the photovoltaics into alternating (AC) single-phase or triple-phase current. The AC current is then fed into the grid of the local utility or power grid operator. (In the text which follows, "utility" will be used to refer to both the utility company and the power grid operator.)

Large, free-standing photovoltaic systems often have an installed power of at least 50-100 kWp up to the megawatt range, fig.55 and generally feed the electricity into the medium-voltage grid. Their main applications are for industry, municipalities or companies in the power supply sector. Depending on the location and system configuration, the systems can be operated not only to supply electric power but also to meet peak loads or stabilise voltage levels. The spectrum of system operators includes utilities, industrial companies and local government authorities. The trend specifically in Germany is toward very large roof-installed photovoltaic systems up to a rated power of two megawatts and free-standing arrays up to four megawatts.

EMBED Word.Picture.8

Fig.54. Schematic diagram of a small, grid-connected photovoltaic system on a private house (Image: Solarpraxis AG, Berlin, Germany).

Fig.55. Free-standing, large grid-connected photovoltaic system ("Sonnen", in Germany) with a rated power of 1.75 MW (Photo: Voltwerk AG, Hamburg, Germany).Fig.56 and fig.57 show configurations for feeding electricity into the public and an intermediate grid.

Fig.56. Configuration for feeding electricity directly into the public grid (Image: Solarpraxis AG, Berlin, Germany).

Fig.57. Configuration for feeding electricity into an intermediate grid (power transit) (Image: Solarpraxis AG, Berlin, Germany).

16. Management and monitoring of exploitation of photovoltaic systems

Grid-connected photovoltaic systems have achieved a high quality standard. Depending on the type and the manufacturer, performance guarantees for 15 to 25 years are now given for photovoltaic modules. Thus, they represent long-lived investment goods, which can operate well for 30 years or more, independent of the formal guarantee.

Inverters have developed from manually constructed, individual units to mass-produced industrial products, which leave little to be desired concerning their technical standard and efficiency. Nevertheless, the inverter still has weaknesses concerning its long-term reliability. Development work is primarily directed toward improving reliability and lifetime. In practical system operation, the lifetime can already be influenced positively by choosing a "cool" position for the inverter.

It is highly recommended to include system monitoring or control in systems with several independent subsystems, as reading the value from one central supply meter is a very inaccurate method of diagnosing a fault in a single subsystem. An obvious approach is to use the data logger or monitoring device offered as an option by various inverter manufacturers.

The typical, specific energy yield of a grid-connected photovoltaic system depends, among other factors, on site-specific characteristics, the quality of the components used and the care taken during installation. Fig.58 shows the typical energy flow diagram for a grid-connected photovoltaic system. The ideal energy yield would be obtained if the incident solar energy were converted to electricity according to the specifications on the module data sheet. However, other factors also play a role in a real system.

The basic principles of the photovoltaic conversion process mean that the module efficiency decreases logarithmically with decreasing irradiance, and reaches a value of zero when it is completely dark. Depending on the frequency distribution of the irradiance in the module plane, the reduction amounts to about 4.5 %. The reduction due to soiling is strongly dependent on the location. Industrially polluted regions must reckon with larger values than the 2.5 % indicated here. The type of mounting affects the operating temperature. Free-standing systems or roof-mounted systems with very good ventilation of the back surface experience a temperature-dependent annual reduction of about 3.5 %. However, this value is significantly greater for roof-integrated systems with limited back-surface ventilation.

The choice of the cable cross-section and the additional installation material used (switches, separators, fuses, etc.) causes circuit losses which increase quadratically with the current. When the cable cross-section is being chosen, a voltage drop of 1 % in the circuit has become accepted as a compromise between investment costs and energy losses. The possible mismatch between individual strings seldom causes a problem, as systems are usually constructed of modules from one production batch.

The losses due to the inverter have two components, the losses involved in the direct DC/AC conversion and the losses resulting from the non-ideal setting of the operation point at any time. Finally, when the losses on the AC side and in the meter are also taken into account, the real amount of electricity fed into the grid is obtained.

Specific energy yields of many systems have been determined within long-term monitoring programmes. To allow comparison between different systems, a universal quality criterion has been introduced, which depends only weakly on the location. The so-called "Performance Ratio (PR)" is the ratio of the real energy yield (e.g. electricity fed into the grid) to the ideal energy yield. In the example shown in fig.58, this is

The performance ratio can also be represented as follows:

Fig.58. Energy flow diagram for a grid-connected photovoltaic system (rated power of 1 kW, located in Northern Germany) (Image: Solarpraxis AG, Berlin, Germany).Recently installed systems at good locations in Germany achieve specific energy yields ( = energy yield relative to the installed electric power rating) exceeding 930 kWh / kW. The Performance Ratio of good systems is then well above 70 %. The PR value is particularly informative if the ideal energy output or the specific energy yield is based on the measured rated power of the modules. Otherwise, of course, it is easy to influence the PR value in the desired direction by the "choice" of the reference value.

As an example, the operating results of a well-functioning, small photovoltaic system (1 kW) are presented in fig.59 by the evolution of the monthly PR over 3.5 years. The free-standing generator on a flat roof is oriented to the south and has a tilt angle of 32 . The increased temperatures during the summer months cause a slight reduction of the PR, as do the reduced irradiance values in winter.

A further example is given by fig.60, which shows the PR of a 100 kW grid-connected photovoltaic system from Switzerland, which started operation in 1989 and has been monitored in detail. The results obtained have been very good on average; operation was disturbed in 2000/2001 due to inverter defects.

Fig.59. Monthly performance ratios over 3.5 years for a recently installed, grid-connected photovoltaic system (1 kW) in Northern Germany. Operation commenced in January, 1998. (Data Source: IEA Performance Database, Image: W. Knaupp, Stuttgart, Germany).

Fig.60. Monthly performance ratios over 13 years for a 100 kW, grid-connected photovoltaic system in Switzerland. Inverter difficulties occured in 2000 and 2001 (Data Source: IEA Performance Database, Image: W. Knaupp, Stuttgart, Germany).

If the planning is good, the components are appropriately selected and the installation is carefully carried out, grid-connected photovoltaic systems operate extremely reliably for many years. Also taking the low maintenance demands into account, they present a promising option for power supply, now and in the future.

Management of PV Systems

The management of PV systems is focused on setting the stage of successful PV installation, based on safety and economical analysis. All the components of the installation safety plan have to be identified, also analysis of different utilities interactions, commissioning plan, cost and financial analysis of the system have to be performed.

Main activities :

Critical safety issues of DC power circuit, especially DC arcing and arc-fault issues.

Permitting and inspection processes for PV systems.

Common metering technologies and rate structures for PV systems.

Different types of utility connection agreements.

Commissioning process of PV systems inspection, maintenance, monitoring and successful troubleshooting.

Introducing the sources of costs, associated with PV systems.

Methods for calculation of investment return.

Example

The PV system parameters are :

Analyzed period 1 year.

Measured solar irradiation IR = 120 kWh/m2.

Solar generation area A = 10 m2.

Efficiency factor of the PV modules = 15 %.

Electrical energy exported to grid Egrid = 110 kWh.

Calculate performance ratio ( PR ).

Total PV plant output is

E = IR . A = 120 . 10 = 1200 kWh. This is the maximal available energy for measured irradiation. The nominal output is :

Enominal = E . kWh.The performance ratio is :

PR = Egrid / E nominal = 110 / 180 = 0,61 or 61 %.

REFERENCES

[1] SOLTRAIN Study materials, Volumes A,G,C, Technical University of Varna, Bulgaria, 2004.

[2] Kipp & Zonen. http://meteostanice.agrobiologie.cz/figs/cm11.pdf.

[3] PVGIS, http://re.jrc.ec.europa.eu/pvgis/.

[4] PVGIS, http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

[5] Michael C. Baechler Theresa Gilbride, Kathi Ruiz, Heidi Steward, Pat M. Love. Solar Thermal & Photovoltaic Systems, June 4, 2007.

[6] Solar Energy : Thermal, Mihir Sen, University of Notre Dame, November 6, 2009.

[7] USA Energy Information Administration (EIA).

[8] Peuser F.A., K.H. Remmers, M.Schnauss, Solar Thermal Systems, SOLARPRAXIS, Berlin, 2002.

[9] Duffie John A., W.A.Beckman, Solar Engineering of Thermal Processes, John Wiley & Sons.

[10] http://www.schott.com/uk/english/download/solar_thermal_rd319.pdf.

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