Final Year Honours ProjectFor the Degree of

B.Eng. in Mechanical Engineering with Energy Engineering

Journal Paper

Fraser StalkerH00104446

Techno-Economic Analysis of Solar Thermal Technology

May 2015

Project Supervisor: Dr. T.S. O’Donovan

School of Engineering and Physical Sciences

Mechanical Engineering

Techno-Economic Analysis of Solar Thermal TechnologyFraser Stalker

School of Engineering and Physical SciencesHeriot Watt University, Edinburgh, EH14 4AS, Scotland

fs113@hw.ac.uk

Abstract

The objective of this study is to compare two solar thermal systems - Closed and Open loop - currently in operation at Heriot Watt SISER Test Site. As energy prices and environmental awareness increase, a critical comparison of various forms of renewable systems is paramount. A simple performance analysis was carried out on the basis of supply temperature from both systems. An economic and sensitivity analysis was then performed on the basis of the system’s relative performance, indicating the viability of each system as an investment. While the Closed loop Navitron system proved more effective, both systems attained positive NPV values over a range of conditions.

Introduction

In a period of increased financial responsibility the interest in renewable energy systems has become apparent. Solar technologies in the UK are often disregarded due to misperceptions regarding productivity, misinformation, lack of trust etc. [1]. While, of course, there is a higher level of solar radiation at the equator than in the UK [2], this does not mean that solar technologies cannot provide a reduction in energy expenditure for a variety of consumers within the UK. Solar Thermal technology has two major applications, ranging from large scale CSP (Concentrated Solar Power) facilities designed to generate electricity to smaller scale systems used for low temperature

water (pre)heating [3].As can be seen in Figure 1, water heating accounts for 25% of domestic energy consumption [4], so any reduction in expenditure could amount to significant savings.

Figure 1 - Share of Domestic Energy

The increased interest has led to government schemes and guarantees in regards to domestic energy generation as well as a rise in attempts by individuals to minimise their impact on the environment. The most obvious example of government’s commitment to a greener energy infrastructure is the implementation of the Kyoto Protocol. The Kyoto Protocol is an international treaty that specifies legally binding commitments to reducing greenhouse gas emissions [5]. The implementation of the Kyoto protocol within the UK has seen the introduction of various policies, most notably for homeowners the Feed-In Tariff (FIT) [6] and Renewable Heat Incentive (RHI) [7]. The incorporation of these tariffs is aimed at making home-generation systems cost-effective for typical households. This, combined with the increasing price of

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fossil fuels and their associated volatility, would suggest the prevalence of home-generation systems will only increase.

Working with two systems previously installed on campus at Heriot Watt, previous students have looked at various aspects of solar water heating. R. Palisse has used the Taguchi Method to analyse the most important parameters affecting output [8], P. Stephens determined a formula for the LCOE [9] and M. Saar has previously used Polysun and experimental data to determine the LCOE of the Navitron system [10].

Using their work as a basis, the Closed loop Navitron and Open loop Soltropy systems can be evaluated. An experimental model will be developed, allowing a comparison of their economic benefits and a sensitivity analysis to be performed. A simulation software, Polysun, will be used for validation.

Literature ReviewA study of available material revealed a lack of objective comparison between Closed and Open loop solar thermal systems. The majority of scientific papers focus on the collector debate, with only personal accounts detailing the benefits of either system. Performing an unbiased analysis of these systems under common conditions should prove beneficial to future advances in the field.

Collectors A variety of solar collectors are available for solar thermal systems. However, their uses vary between hot water and electricity generation.

A variety of factors including shading, tilt and orientation factor into the placement of a collector, typically in the UK it is a 30°

tilt facing south that receives the largest amount of solar radiation (see Appendix 11).

The two types of collector typical in residential systems and installed on-site are discussed.

Evacuated Tube Collector

Figure 2 - On-Site Evacuated Tube Collector

This form of solar collector was employed in both the Navitron and Soltropy systems. The collector is a 30 Tube 58mm Solar Panel (SFB30-58) from Navitron, although Soltropy modified the existing collector to allow for a direct heating system.

Figure 3 - Evacuated Tube Collector

Figure 3 displays the method through which heat transfer occurs in Evacuated Tube Collectors. Each tube consists of two glass tubes made of strong, hail resistant borosilicate glass. A vacuum is formed between the two tubes, significantly reducing heat losses. The inner tube is coated in a selective heat-absorbent

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coating, which makes use of the ‘Black Body Heat Principle’ [11] [12]. The solar radiation incident upon the tubes is absorbed and the heat transferred into a working fluid within the pipes.

As the heated fluid rises it heats the secondary fluid within the manifold - water in the Soltropy system and an anti-freezing mixture in the Navitron. When the temperature of the working fluid is greater than that of the tank temperature, a pump is automatically switched on, transferring the heat from the manifold/system to the tank. The working fluid within the pipes then cools down and falls to the bottom and the cycle begins again.

Typically Evacuated Tube Collectors are regarded as the most efficient form of collectors due to a variety of factors [13] [14]. Their circular shape allows for ‘collection’ at a wide range of angles, allowing for production at non-optimum early morning and evening hours [15] [16]. The vacuum within the tube practically eliminates associated convection and conduction heat losses and allows steady production regardless of outside temperature or wind effects [17]. The modular nature of individual pipes allows for quick and easy replacing should a pipe be damaged. The only drawback of this system is the higher cost.

Flat Plate Collector

The second form of collector installed on-site was a Flat Plate Collector (FPC). The intended use is in the newly installed SunAmp system, which unfortunately due to a delay in installation will not be examined in detail.

Figure 4 - On-Site Flat Plate Collector

Typically, Flat Plate Collectors consist of an aluminium frame (although more expensive fibreglass and steel frames can be used), an absorber and a transparent solar glass or polycarbonate cover. The rear and sides are clad in insulation to reduce conductive losses and the cover is designed to transmit the maximum amount of solar radiation, to generate a miniature ‘greenhouse’ effect and to prevent losses due to convection, particularly in windy climes.

Figure 5 - Flat Plate Collector

The absorber within a Flat Plate Collector is typically a thin sheet of a thermally stable material, most often copper due to its anti-corrosive properties and high level of heat conductivity [18]. A grid or coil of heat-transfer fluid (again typically a water or anti-freeze mixture) is pumped through the system, drawing heat from the absorber to the tank. It is also possible to have open and closed loop flat plate collector systems, in the case of SunAmp it is a

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Closed loop, phase change heat battery system.

While Flat Plate Collectors are generally not considered as efficient as Evacuated Tube Collectors, they are cheaper [19] [20]. The strong cover provides a very durable and resistant form of protection. Flat Plate Collectors also benefit from an aesthetic perspective, and are more easily integrated into the fabric of the building [21], making them a more aesthetically attractive alternative to Evacuated Tube collectors, particularly to developers who are looking to sell on the property. In winter conditions the heat losses through conduction in a Flat Plate system may be significant due to the difference in ambient and operating temperatures.

Figure 6 - Integrated FPC

Evacuated Tube Collectors v Flat Plate Collectors

As stated, both forms of collector are typical in residential systems. There are proponents for each system, with persuasive arguments for each. From a purely technical standpoint, it would appear that the Evacuated Tube collector should dominate the market. The cost of maintenance and replacing damaged tubes is much less, the associated heat losses are minor in comparison, the output in low-angle incident solar radiation conditions is higher and their performance in cooler environments lends itself to a wider

market. However, their higher cost is a significant drawback.

The greater energy production and other benefits appear to make the ETC collector a more attractive investment. However, accounting for the Renewable Heat Incentive and lower capital cost, arriving at a definitive conclusion on Flat Plate v Evacuated Tube is difficult.

Open and Closed Loop SystemsThe Navitron and Soltropy systems make use of a similar form of heat storage, a hot water cylinder, although the nature of heating varies.

Open Loop Soltropy System

Figure 7 - Polysun Soltropy System Diagram

The Open loop Soltropy system operates with a working fluid of water and as such is generally applied in temperate climates to avoid freezing. The water in open loop systems is pumped through the manifold and extracts the heat from the solar panel, the heated water then enters the tank and flows from the taps/shower head.

Due to the circulatory nature of such a system, the water within the tank tends to be well mixed and have a relatively constant temperature from top to bottom. Generally regarded as a cheaper option with slightly better efficiency and can be easily integrated into existing systems. However, the water generally does not reach the temperatures of a Closed loop system.

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Closed Loop Navitron System

Figure 8 - Polysun Navitron System Diagram

An antifreeze mixture is pumped through the manifold and through use of a heat exchanger, heat is transferred from manifold to the fluid in the water tank.

The placement of the heat exchanger causes stratification of the water within the tank. Stratification is the separation of a fluid into horizontal layers, in this case due to temperature [22]. This is displayed graphically in Appendix 10.

This system is favourable in colder environments where freezing is possible. Typically attains a slightly lower efficiency at a greater initial cost, although increased temperatures at higher stratification levels may prove more useful.

Auxiliary heating would theoretically be used in both systems, to raise the temperature of the water to the desired level. If supply temperature is greater than desired, then a mixing ratio is applied to lower it.

Health and Safety Concerns

Legionnaires’ disease, a form of bacterial infection, can be contracted from stagnant sources of water held between 20-45°C, temperatures typical within the hot water tanks. Raising the water temperature to >60°C for a period of one hour each day is sufficient to reduce bacteria levels [23]. As shown in Appendix 10, the Closed loop

system is more likely to attain these safety temperatures, however auxiliary heating would almost definitely be required.

While there appears to be a consensus on the relative efficiency of each system, the basis of the argument would benefit from being focused on the effectiveness of each system. The nature of heating – stratification v full tank – may prove to cause a significant change within the effectiveness of each system.

Should a large volume of demand occur, the stratification within the Closed loop system may lead to use of the lower temperature water, reducing the effectiveness of the system. The Open loop system, with its relatively constant temperature, may cope with a larger volume of demand more effectively. The temperature of supply water would remain constant, providing identical savings per unit volume from the top of the tank to the bottom.

Load Profiles A hot water load profile is an indicator of how the hot water demand varies over a given time period. However, domestic hot water demand is very unpredictable, with numerous sources of variation. The most common demands are for showers, baths, clothes washing, dish washing, sinks etc. which are discrete events and often do not occur at set times from day to day. Combining this with the range of households, occupancy, differing habits etc. it becomes very difficult to define an ‘average’ household profile. Therefore it is necessary to consider how each system might operate under a variety of load profiles.

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Figure 9 - 'Average' Hot Water Demand Profile

A study of load profiles indicates several common factors;

Regardless of the day of the week and number of people, there is a peak in demand early in the day and in the evening. This can be associated with showers, cooking, cleaning and increased activity.

Day of week, and implied altering of schedule, has both an effect on the volume of demand and shifts the time at which it occurs. Weekends typically have a larger, more sustained demand as the occupants would more likely remain in the house. The morning demand also tends to shift a number of hours into the late morning. As expected, an increase in number of occupants increases demand.

Where the ‘average’ load falters most significantly is the sustained demand throughout the night and afternoon. This is most likely due to an accounting for nightshift workers or the atypical household. In reality the majority of households would have no demand throughout the night and none/minimal during the day on a weekday, presuming all occupants were outwith the household.

The two systems being studied are subjected to a known load profile. Two

further theoretical load profiles were developed to be used in the experimental model. All three profiles can be found in Appendix 16.

Uses The breadth of applications highlights the viability of solar thermal as a source of renewable energy. Currently, solar photovoltaics have a much higher profile than solar thermal systems, although this is primarily due to lack of consumer knowledge. While solar photovoltaics offer a more flexible product – electricity – the efficiency of a solar thermal system is significantly greater [3] [24] [25] and the area required for installation is significantly less [24] [26]. This combination makes them prime investment opportunities for homeowners. However, their suitability for domestic use has not prevented their use in larger scale applications.

Industrial Case Study - Ivanpah Solar Thermal Facility

Located in the US it demonstrates the concept on a much larger, industrial scale, although the practice they have applied is slightly different. Through use of a field of solar concentrators solar radiation is focused on the pipes of a central water cylinder, thereby heating the water within and attaining the temperature required to produce steam, which in turn spins a turbine and generates electricity. This large scale project is the largest example of solar thermal theory. The capacity is roughly 377MW, enough to power 140,000 Californian households and preventing roughly 400,000 tons of CO2 emissions compared to its’ equivalent fossil fuel generation [27] [28] [29].

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Figure 10 - Ivanpah Solar Thermal Facility

However, the facility has encountered fierce opposition throughout its development stage (finishing construction in 2014) and continued scepticism in regards to its co-existence within the Mojave Desert as well as suffering from poor initial performance. Numerous arguments have been raised about its large area (roughly 14.2 square km [27]) and its effect on local eco-systems [30]. Whilst portrayed as a green source of energy, it does suffer opposition from wildlife proponents due to the fact numerous birds have been killed by the focused solar rays [31]. Reports vary as to the scale of the problem, with low estimates of 1,000 birds per year reaching 28,000 per year according to an expert for the Center for Biological Diversity environmental group. In perspective, however, some 175 million birds per year in the US die due to flying into power lines [32]. The reflections from the sea of heliostats may appear as water to birds from a distance, and the intense light can attract a myriad of insects, a major food source for many birds, so current studies have no way of accurately predicting how this new system may affect future migration patterns etc.

Initial predictions regarding the output of the facility estimated one million plus megawatt hours per annum [33], however over the initial eight months of operation have only produced 254,263 megawatt-hours of electricity [34], roughly 38% of expected production. This has been blamed on poor weather and teething errors based

on the unprecedented scale of the project [33].

Solar thermal has also seen widespread use in smaller, internal, commercial systems. In a variety of sectors, water heating is a significant expense and with “a raft of European Directives, national legislation and government policy across the domestic and commercial building sectors now existing to reduce the long term energy and carbon impact of our buildings on the environment” [35] it is an obvious target for reduction.

Common uses in commercial settings are public swimming pools, fisheries, catering facilities and washrooms. A company, Solflex, has installed three examples of where solar thermal can provide significant cost and carbon emission savings.

Commercial Case Study 1 – Bowland Wild Boar Leisure Park

Figure 11 - Bowland Wild Boar Leisure Park

At Bowland Wild Boar Leisure Park electricity was used to heat water due to the remote location, therefore two evacuated tube collectors to an area of 4sq m were installed generating approximately 2,000kWh annually and supplying up to 50% of the required hot water [36]. Solely accounting for the non-domestic RHI tariff[37], this would amount to £212 savings, which combined with the fuel savings will amount to a significant sum.

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Commercial Case Study 2 – Shallow Grange Camp Site

Figure 12 – Shallow Grange Camp Site

Similarly, Shallow Grange Camp Site, Buxton, Derbyshire were using LPG to provide hot water for showers and underfloor heating, so installed 12.48sq m of flat plate collectors [36]. The annual production of 3, 910kWh alone would provide £414.46 income from the RHI and the system is capable of providing 55% of annual need and 100% during peak summer times, where usage can be expected to be at a maximum.

Commercial Case Study 3 – Ryedale Public Swimming PoolWhen performing a scheduled boiler replacement, Ryedale Public Swimming Pool opted to incorporate a solar thermal system into their plans [36] [38] [39]. 21 evacuated tube collectors were installed and a reduction of 46% per annum on the cost of heating the 250sq m community swimming pool was noted.

Figure 13 - Ryedale Public Swimming Pool

Most often, however, solar thermal technology is employed in residential scenarios. The technology to transport heat is extremely inefficient so having the system ‘on-site’ is where the biggest advantage can be found. As mentioned, heating water accounts for roughly 20-25% of domestic energy consumption and as such any means of reducing the costs associated are desired. Typically, solar thermal systems are said to be capable of reducing annual hot water bills by roughly 60% [35] [40] although this obviously varies significantly from summer to winter.

While this highlights the fact solar thermal is incapable of solely providing the required heating, which may appear unappealing, there is no doubt it is a worthy method of cost reduction. It has been applied in a variety of residential scenarios from individual households, social housing, private developments, new builds etc. but the target market is off-grid houses where the fuel for heating is LPG, oil or electric as these cost significantly more than gas [41].

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TheoryThe governing equation related to solar hot water systems is simply [42];

Qaux=mC p(T d−T s)

This equations defines the required energy required to raise the supply water from supply to demand temperature. The savings generated by the system occur when the supply temperature is raised through addition of solar energy. This reduces the amount of energy required from an auxiliary heating source i.e. boiler or immersion heater. Due to the working process of a gas boiler or other such methods (i.e. heating up a store of water and letting it ‘cool down’ until use or the next heating period), the comparison was drawn on the presumption of instantaneous electric heating.

The amount of useful heat energy provided by the solar system can be quantified as such;

Quse=Q solar−Q losses−Epump

Where Qsolar is the heat provided by the panel, Qlosses consists of internal losses due to pipework, tank losses and heat exchanger efficiencies. Epump denotes the energy required to pressurise the system in order to transfer the heat from manifold to tank.

Using Polysun, pump energy was found to be 14kWh per year, in the region of 0.5-1%.

Despite insulation the hot water cylinders on-site, without solar input, have a standing heat loss of 1.96kWh per day (taken from technical specifications), totalling 715.4kWh per year. Accounting for the solar heating, losses from the tank itself would be very difficult to quantify,

as the passive heating of the water serves to negate or reduce this loss. For the sake of the initial calculations, piping losses are taken as equal to the reduction in tank losses due to the solar heating effect.

A full costing was done by Mira Saar [10], with the cost of the Navitron installation and relevant certification amounting to £3,474.40. The Soltropy system was a modified version of the Navitron, so exact costing would be difficult. However, due to the reduced complexity and number of components, as well as easier integration into existing water systems, it would be less costly in reality. For the sake of calculations it was taken as identical.

Typical maintenance costs have been debated, as well as degradation rates. Ideal/expected maintenance costs for the Navitron system amounted to 0.69% of initial expense per annum. This includes planned pump and anti-freeze mixture replacement in the Navitron system. Accounting for unexpected maintenance, this is in relative accordance with the 1% stated for solar systems [43]. With the Soltropy system some water treatment and freezing-damage repair would fall within the same range, although values are difficult to quantify. Therefore 1% was deemed appropriate for both systems.

Degradation rates vary, with numerous case studies reporting different rates. Due to the unavailability of a proven figure, degradation rate was set as a nominal 2% per year, to a maximum degradation of 80% initial performance.

The availability of grants for solar thermal systems is the subject of much debate. Research into this topic proved inconclusive, therefore grants were excluded from the calculations. It was assumed as a cash purchase, with a salvage potential of 35% initial purchase price [44].

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The calculations were carried out on the current RHI tariff (19.2p/kWh) and assumed that at the end of the guaranteed 7 year tariff, it would end rather than continue at a lower rate.

The Net Present Value (NPV) [45] [46] was determined as an appropriate method of ascertaining if investing in a solar thermal system was both profitable and preferable to other forms of investment. NPV dictates whether or not an investment is capable of improving the return on initial capital in comparison to a known investment. The formula for obtaining the NPV is as follows;

NPV =−CF0+CF1

(1+i)1 +CF 2

(1+i)2 +…+CFn

(1+i)n

Where CF0 represents initial investment, CFn both cash inflows and outflows over the life of the project ‘n’ and ‘i’ the discount rate. The discount rate is calculated in reference to the risk of the project, and is commonly regarded as the rate of return on a ‘safe’ or known investment. If the NPV of a project is greater than 0 this indicates that it is projected to be more profitable than the ‘safe’ investment. If it falls below 0 then the investment should be avoided.

Using Polysun’s database as a reference, cold water supply temperatures were taken as 13°C for August and 12°C for September.

A number of other factors were varied when using the generated model, including electricity price, energy inflation, general inflation and NPV discount rate in an effort to attain a range of outcomes.

The base values used, while not the variable being altered, were 14.05p/kWh

for electricity, a 4% discount rate for NPV, 5% energy inflation and 4% general inflation.

Methodology Local weather data was taken from the W8681 Solar WATSON weather station installed on site, gathering a range of data such as solar global radiation, wind, rain etc. Unfortunately this system does not measure the diffuse radiation, a major component of solar heat generation. Therefore an approximated version of De Jong’s Model of Diffuse to Direct Radiation [47] must be applied in order to determine the distinct components.

An Arduino based Data Acquisition Module (DAQ) was used to record and automatically log the temperature at various points throughout the entire system, most notably at eight points within the tanks (shown in Appendix 14). An Arduino system was also used to control the water outlet, simulating loading conditions.

Sample data was taken from two weeks in 2014 in order to perform the initial analysis. These data arrays were then used to perform an analysis on the performance of the systems and their respective potential savings. Applying De Jong’s Model, it was possible to upload the local data into the Polysun database.

The values obtained were then compared to those outlined by Polysun, and a discussion formed on the potential flaws and limitations. A cost and sensitivity analysis was then performed to determine how the systems would behave under various conditions.

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Results & Discussion

Trend Analysis Data was obtained from the Arduino DAQ modules via an SD card. The data was recorded from 17/07/2014, although upon initial investigation the data was erratic and did not appear to follow a predictable pattern;

Figure 14 - Initial Weather and Soltropy Tank Data

After shifting the focus to data from August and September - and upon discussion with Dr. O’Donovan - it became apparent the system was merely approaching a steady state phase. While typically expected within a new system, this would also have been due to the initial manual nature of ‘loading’ i.e. the hot water was initially let out by hand, before the current automatic control system was implemented.

Highlighted in Figure 14 is an indicator of the system not performing as expected. Despite solar radiation –albeit a relatively small amount – the volume of water within the tank does not experience a temperature rise, in fact it decreases. Combined with the consistent decrease in tank temperature, it may be suggested that the

tank is finally reaching a steady state. Towards the end of the week shown, the tank operates as expected, with solar radiation directly affecting the temperature of the tank. The offset of peak solar radiation and peak tank temperature is apparent, implying there is a delay in heating.

Following this discovery, although it meant moving further into the year and potentially diminished solar radiation, it was decided to use the weeks 11th-17th of August and 8th-14th of September. This avoided any issues regarding steady state conditions and it is hoped that using data from these periods will also indicate the viability of the system in autumn/winter months.

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Figure 15 - Weekly Tank Temperatures and Radiation Data

Figure 15 highlights the expected trends, while also revealing more information on the two systems. As can be seen, and as was expected through the literature review, the Navitron (Closed Loop) system, reaches higher supply temperatures than the Soltropy (Open Loop) system, directly related to the incident radiation. However, it is worth noting that the Navitron system also falls below the temperature of the Soltropy system overnight. This was investigated further, highlighted in Figure 16;

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Shown in Figure 16 is an indication of the disparity in possible effectiveness of both systems. The supply temperatures of both systems begin to rise as the solar radiation is applied to the collector. However, the Navitron system falls to a lower temperature once radiation ceases

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i.e. at night/early morning. This may suggest the Soltropy system may be more useful in early hour loading scenarios.

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Figure 17 - Relative Energy Savings

Through use of the model generated in Excel and analysis of the respective supply temperatures, it was possible to estimate the amount of energy saved in heating the water to the desired temperature. As shown in Figure 17, as solar radiation increases the temperature of fluid within the tank (reaching a peak in the late afternoon), it can be seen that the percentage savings follow a similar trend. The highest relative savings occur at the period of peak radiation. However, this has the potential to be very misleading;

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Figure 18 - Real Savings

Shown in Figure 18 is the most notable discovery of the project; savings are directly related to both timing and volume of demand. Despite peak relative savings occurring at roughly 15:00 in Figure 18 (a period of low demand), peak real savings can be seen to occur at the periods of highest demand in Figure 19.

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Figure 19 highlights the Load Volume-Savings relationship. The highest loading period occurs during the morning, with a raised demand also occurring between 16:00-18:00. The highest real savings occur directly as a load is drawn from the tank. As the tank has gained heat by the time the evening demand occurs, this is when savings are greatest.

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Figure 20 follows on from the Load Timing-Savings relationship, indicating the importance of tank temperature/load timing to savings. The highest temperatures occur within the tanks during periods of low demand, with the peak load in the morning occurring before the tanks have begun heating.

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Summary of Trends

Highest tank temperatures are obtained as a direct result of solar radiation, although with a delay

The Navitron system attains higher temperatures throughout the day, but tends to fall below the temperature of the Soltropy system overnight and in the early hours of the morning

Relative savings are directly related to solar radiation i.e. largest relative savings occur when radiation/tank temperature is highest

Real savings are directly related to the volume and timing of demand , as well as the supply temperature

Discrete Cost and Performance Analysis Using the model generated, it was possible to compare the systems on a discrete basis. Using an identical formula and loading profile, the energy savings for each system were calculated. Applying the constraints detailed in the Theory section, the following results were obtained;

Figure 21 - Weekly Savings

Solar radiation levels were higher during the chosen September date range than the August range, explaining the increased savings. Combined with the lower temperature water source (~13°C in August and ~12°C in September taken from Polysun), this indicates the unexpected viability of solar thermal in winter months. Due to the increased supply temperatures obtained within the tank, and the nature of the loading, the Navitron system appears to provide a higher level of savings.

Very noticeable is the contribution the RHI makes to the cost savings. Due to mistrust towards the Feed-In Tariffs ever-changing

rate, this may put many people off the proposition of investing in solar thermal systems. However, as shown later on, varying the value and duration of RHI tariffs still allows solar thermal to be profitable.

Despite the obvious inaccuracies with extrapolating this data over the year (with the level of average radiation varying, typically in a predictable manner i.e. larger amount of solar radiation in summer than winter), doing so would allow a very rough comparison to be drawn between this model and the values obtained through Polysun.

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Figure 22 - Annual Extrapolation v Polysun

Due to licensing issues, the Soltropy system could not be accurately modelled. As the tank was modified from an existing heat exchanger type, it did not exist on the SunAmp database. This required the use of the nearest alternative suitable for Open loop use. This was a larger tank with a larger associated demand, a variable that could not be altered without the Developer version of Polysun. This lead to skewed results for the Soltropy system.

The accuracy of the Navitron model was encouraging, however, with a maximum of 16.12% disparity. The accuracy of the energy performance was within 10%, more than acceptable for experimental standards.

A proportion of the differences in Figure 22 must be attributed to the rough extrapolation. Quantifying the error introduced by the extrapolation would be impossible without access to a full year of meteorological and tank temperature data.

Contributing to the inaccuracy could be a raft of manufacturing and assembly errors, particularly within the Soltropy system as this was a Navitron system retrofitted to perform as an Open loop system. This required modification to both the tank and collector, potentially damaging or altering insulation, vacuum seals and a number of other factors which could affect performance.

Issues with the precision of the data readings and the method through which they were discretised (five second readings

for system temperatures or one minute readings for meteorological data, averaged over an hour) would account for a further source of error.

The most obvious issue occurs with the differing results on ‘best solution’. The model developed indicates due to the time and relatively low volume of demand, the increased temperature caused by the stratification within the Navitron tank results in a better return. However, this is contradicted by the simulation software. This is most likely due to the higher volume of demand in the Soltropy simulation, a factor already discussed.

During analysis it was also noted there was a wiring error, affecting Soltropy and Cold Water supply temperature readings. These supply readings matched those of the Navitron system. While the cold supply temperature was determined from Polysun, the Soltropy supply temperature was obtained as an average of layers within the tank. While this principle may hold for a Open loop system i.e. the temperature should be constant throughout the tank, in practice it no doubt introduces another source of error.

Extrapolating the total annual demand results in a value of 2,692kWh. The Navitron system would appear to reduce annual water heating bills by 40.1% and the Soltropy system by 30.3%.

18

Load Profiles & PerformanceUpon applying the theoretical load profiles, the effect on both performance and NPV was obvious. Both systems increased their energy savings and provided a greater NPV as expected. The Student household especially, with its afternoon and evening demand, managed to obtain significant savings.

However, it was noted that this does not account for the greater volume of demand within the tank i.e. the tank temperature should suffer as a direct result of hot water demand. As such, while the exercise of applying various loads to the model definitively proves the benefit of shifting load to high temperature periods, it is impossible to be confident in the figures obtained. The results are shown in Appendix 17 and Appendix 18.

Lifetime Performance and NPV Using the ‘base’ values indicated in the Theory section, it was possible to obtain the following figures;

Figure 23 - Lifetime Values

Figure 23 accounts for all mentioned factors within the system, including energy and general inflation linked costs. It states that both systems, under the operating conditions stated, are profitable investments providing greater value than the nominal 4% discount rate. A full range of sensitivity analyses can be found in Appendices 1-9, detailing how a variation in each factor can alter the NPV of each system.

While NPV provides a very useful form of evaluation, the majority of prospective purchasers are more interested in the payback period;

Figure 24 - Payback Period

These results are obtained under the base conditions, although are heavily influenced by the assumption of equal initial cost. In reality the Soltropy system would be cheaper.

19

The salient results obtained from the sensitivity analyses are as follows;

• The Navitron system is insulated to changing electricity prices, capable of retaining a positive NPV at below 8p/kWh, nearly 50% of current prices, and at an energy inflation rate of 0%. The Soltropy system becomes a negative investment if current prices drop below 12p/kWh or 4% energy inflation.

• The Navitron system does not rely on the RHI scheme to attain a positive NPV, whereas the Soltropy system requires five or more years.

• Investing in the Navitron system is equivalent to over 7% investment, according to the NPV calculations, whereas the Soltropy is almost 5%.

• Surprisingly, both systems maintain a positive NPV with a maximum degradation factor of 50% (occurring at 2%/year).

• Were the proportional maintenance to increase significantly above the 1% stated, both systems will suffer a negative NPV. Soltropy between 1-2% and Navitron at roughly 3%.

• Were the initial system to cost £4,000, Soltropy would be a roughly equivalent investment (at 4% discount rate), although even at £5,000 the Navitron system remains positive.

Carbon Savings

While the majority of this study focuses on the economic aspect of solar thermal heating, it also serves the dual purpose of highlighting the potential reduction of CO2 emissions associated with grid-electricity production. Under the ‘base’ conditions, the lifetime CO2 savings are shown in Figure 25;

Figure 25 - Carbon Savings

Using data obtained from the US Environmental Protection Agency [48], the savings shown in Figure 25 equate to eliminating 0.85 and 1.09 of an average, annual household’s CO2 emissions respectively over the lifetime of the system.

ConclusionA performance and cost analysis of two distinct solar water heating designs has been completed. As initially expected through the completion of a literature study, the Navitron (Closed loop) system appears to provide a better investment,

20

resistant to numerous variable effects. Both systems attained a positive NPV under the determined conditions, with the Navitron system achieving £1,619.48 and the Soltropy (Open loop) system £392.14. Under the conditions within this experiment, clearly the Navitron system was more effective. However, with a more demanding load profile, it would be interesting to see how the two systems performed.

The Navitron model performed exceptionally well, although due to factors discussed, the Soltropy system will require further review in order to frame a valid argument. Furthermore, through the analysis of experimental data, several major factors in solar heating have been highlighted. Volume and timing of demand have been noted to have the most significant result on the effectiveness of a system. Unexpectedly, the systems may perform favourably under identical conditions in winter rather than summer, due to the lower water source temperature.

Each system was capable of providing a substantial contribution to annual energy consumption. Navitron provided 40.1% while the Soltropy system 30.3%. While lower than the figures obtained in the Literature Review, these are still significant savings. Combining their financial merits and the accompanying carbon emission savings, it would appear solar thermal systems have great potential moving forward.

Further WorkFurther work with a variety of loading profiles would provide greater insight into how best to utilise these systems. To do so, the two different loading profiles have been coded in Arduino, hopefully to be used by future students.

Correcting the wiring issues and simplifying the DAQ module would also provide significant benefits to future users.

The SunAmp system, briefly mentioned, makes use of a Phase Change Heat Battery. Use of this form of heat storage has the potential to significantly reduce heat losses and eliminate any risk of Legionnaires Disease. Compounded with the space savings it generates, it is possible this method may become the most prevalent form of domestic heat storage in the years to come. Due to the delay in installation, it was not possible to work on this system although it would be very interesting to see how it compares with both existing systems.

AcknowledgementsI would like to give special thanks to Dr. Tadhg O’Donovan for his continued support and patience throughout the course of this project. Further thanks must go to Daniel Rylatt and Amanda Hughes for their assistance and advice within the laboratory.

21

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22

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1051-1064.pdf. [Accessed 18 April 2015].[33] Breaking Energy, “Ivanpah Solar Power Plant: Energy Production Falling Well

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[44] S. G. Saxena A, “Potential and Economics of Solar Water Heating,” MIT International Journal of Mechanical Engineering , vol. 2, no. 2, pp. 97-104, 2012.

[45] G. C. I. Lin and S. V. Nagalingam, CIM justification and optimisation, London: Taylor and Francis, 2000.

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[47] M. Bindi, F. Miglietta and G. Zipoli, “Different Methods for Separating Diffuse

24

and Direct Components of Solar Radiation and Their Application in Crop Growth Models,” 9 July 1992. [Online]. Available: http://www.int-res.com/articles/cr/2/c002p047.pdf. [Accessed 19 April 2015].

[48] US Environmental Protection Agency, “Units Conversions, Emissions Factors, And Other Reference Data,” November 2004. [Online]. Available: http://www.epa.gov/cpd/pdf/brochure.pdf. [Accessed 21 April 2015].

All figures accessed 21/04/2015

Figure 1 - http://quantumenergy.com.au/wp-content/uploads/2014/04/piechart_energy_usage_australian_home.jpg

Figure 3

Figure 5

Figure 6

Figure 9

- http://www.newscientist.nl/assets/3921_ivanpah_mingasson.jpg

- http://www.solfex.co.uk/SolarHeat/Solfex-energy-systems-solar-system-references/Tourism-Leisure/

Figure 12 - http://www.solfex.co.uk/SolarHeat/Solfex-energy-systems-solar-system-references/Tourism-Leisure/

- http://www.solfex.co.uk/SolarHeat/Solfex-energy-systems-solar-system-references/Tourism-Leisure/

25

AppendicesAppendix 1

Effect of Varying Electricity Price on NPV

5p/kWh 7p/kWh 8p/kWh 10p/kWh

12p/kWh

14.05p/kWh

15p/kWh

20p/kWh

25p/kWh

-2000.0

-1000.0

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

Soltropy Navitron

Electricity Price

NPV

NPVElectricity

PriceSoltropy

Navitron

5p/kWh -1255.8-

601.081

7p/kWh -891.64-

110.3498p/kWh -709.55 135.017

10p/kWh -345.35 625.749

12p/kWh18.843

3 1116.48

14.05p/kWh392.14

3 1619.48

15p/kWh565.13

5 1852.58

20p/kWh1475.6

2 3079.41

25p/kWh2386.1

1 4306.24

26

Appendix 2

Effect of Varying Energy Inflation on NPV

1 2 3 4 5 6 7 8 9 10

-1000

0

1000

2000

3000

4000

5000

Soltropy Navitron

Energy Inflation (%)

NPV

NPV

Energy InflationSoltropy

Navitron

1% -498.0 420.12% -320.8 658.83% -116.6 933.94% 119.2 1251.75% 392.1 1619.56% 708.5 2045.77% 1075.7 2540.68% 1502.7 3115.99% 1999.6 3785.5

10% 2578.7 4565.7

27

Appendix 3

Effect of Varying Value of RHI Tariff on NPV

5p/kWh 10p/kWh 15p/kWh 19.2p/kWh

20p/kWh 25p/kWh 30p/kWh-500

0

500

1000

1500

2000

2500

3000

Soltropy Navitron

RHI Tariff

NPV

NPVRHI Tariff Soltropy Navitron

5p/kWh-

328.212648.843

6

10p/kWh-

74.5659990.617

6

15p/kWh 179.081332.39

219.2p/kWh 392.1 1619.5

20p/kWh432.725

91674.16

6

25p/kWh686.371

8 2015.94

30p/kWh940.017

72357.71

4

Appendix 4

Effect of Varying Duration of RHI Tariff on NPV

1 year 2 years 3 years 4 years 5 years 6 years 7 years 8 years 9 years 10 years

-1000

-500

0

500

1000

1500

2000

2500

Soltropy Navitron

RHI Duration

NPV

NPV

RHI Duration SoltropyNavitron

1 year-

433.8333506.524

3

2 years-

288.7695701.989

9

3 years-

146.6661893.466

5

4 years-

7.5231991080.95

4

5 years 128.65921264.45

2

6 years261.8811

21443.96

27 years 392.1 1619.5

8 years519.4434

91791.01

3

9 years643.7839

51958.55

5

10 years765.1639

12122.10

8

Appendix 5

Effect of Varying Degradation Rate on NPV

28

1 2 3 4 50

200400600800

100012001400160018002000

Soltropy Navitron

Degradation Rate (%)

NPV

NPVDegradation

Rate SoltropyNavitron

1%542.515

4 1822.12% 392.1 1619.5

3%323.356

5 1526.8

4%279.068

5 1467.12

5%253.118

1 1432.15

29

Appendix 6

Effect of Varying Maximum Degradation Factor on NPV

0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.50.0

500.0

1000.0

1500.0

2000.0

2500.0

Soltropy Navitron

Maximum Degradation Factor (%)

NPV

NPVMaximum

Degradation Soltropy Navitron90% 609.8 1912.885% 492.5 1754.780% 392.1 1619.5

75%307.254

21505.09

9

70%237.016

51410.45

8

65%183.034

8 1337.72

60%144.421

71285.69

1

55%122.861

3 1256.64

50%117.422

91249.31

2

30

Appendix 7

Effect of Varying Proportional Maintenance on NPV

1 2 3 4

-2500.0

-2000.0

-1500.0

-1000.0

-500.0

0.0

500.0

1000.0

1500.0

2000.0

Soltropy Navitron

Proportional Maintenance (%)

NPV

NPVProportional

Maintenance SoltropyNavitron

1% 392.1 1619.5

2%

-443.049

758784.28

95

3%

-1278.24

207

-50.902

8

4%

-2113.43

437

-886.09

5

31

Appendix 8

Effect of Varying Initial Cost on NPV

2000 3000 3474.4 4000 5000

-1500

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

Soltropy Navitron

Initial Cost (£)

NPV

NPV

Initial Cost SoltropyNavitron

20001724.773

32952.11

3

3000820.9271

62048.26

63474.4 392.1 1619.5

4000-

82.91899 1144.42

5000-

986.7651240.574

1

32

Appendix 9

Effect of Varying Discount Rate on NPV

1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00%

-2000

-1000

0

1000

2000

3000

4000

5000

Soltropy Navitron

Discount Rate (%)

NPV

NPVDiscount Rate Soltropy Navitron

1.00%2635.79

94348.32

9

2.00%1721.14

8 3243.37

3.00% 986.2082348.59

14.00% 392.1 1619.5

5.00%-

91.0294 1021.54

6.00%-

486.533527.922

1

7.00%-

812.425117.665

8

8.00%-

1082.79-

225.661

33

Appendix 10

Stratification within Both Tanks

Each line indicates a thermocouple positioned at a location upon the tank, rising from Point 1 at the bottom to Point 8 near the top. Matlab was used to obtain the graphs.

Appendix 11

Orientation and Tilt

Accessed 21/04/2015 from;https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcQGQwhdAMDmkWAv0tt

NIyshPr3X7ZD8qmQrIGQfReLaOx1mLYiasw

Appendix 12

Example Costing Analysis

Appendix 13

Example Weekly Analysis

34

Appendix 14

Thermocouple Placement

35

Appendix 15

Graphic of De Jong’s Model of Direct to Diffuse Radiation

Accessed 21/4/2015 from;http://www.graceresearch.com/Model

%20of%20Diffuse%20Broadband%20Solar%20Radiation%20under

%20Clear%20Sky%20Conditions%20for%20Web%20Long_files/image003.gif

Appendix 16

Load Profiles12:00:00 AM

02:00:00 AM

04:00:00 AM

06:00:00 AM

08:00:00 AM

10:00:00 AM

12:00:00 PM

02:00:00 PM

04:00:00 PM

06:00:00 PM

08:00:00 PM

10:00:00 PM0

20406080

100120

Load Profiles

Predetermined Load 4S LoadLoad 3P Weekday Load 3P Weekend

Time

Volu

me

of D

eman

d (l)

Three Person Household

Differs from weekday to weekend. Peak in the morning during the week with peak shifted to late morning at the weekend. No use throughout the night with minimal use throughout the afternoon during the week but sustained, minimal-moderate use during the afternoon at the weekend. Peak

36

in the evening at a similar time both weekday and weekend.

Four Student Household

Remains constant throughout the week due to the various schedules of students’ i.e. part-time jobs, sports training etc. Morning and evening peaks with sustained minimal use throughout the afternoons although with potential high volume demands later in the evening or earlier in the afternoon due to working/training patterns.

Appendix 17

Four Student Household Results

Appendix 18

Three Person Household (varied Weekday/Weekends Loads) Results

37