LIFE-CYCLE ENERGY ANALYSIS OF PHOTOVOLTAIC SYSTEMS

By:Suleiman A. Halasah

Environmental Science – The Arava Institute for Environmental Studies

The human race can get a substantial portion of its electrical power without burning fossil fuels or creating nuclear fission reactions through direct conversion of solar energy . 89 petawatts (10^15) of sunlight reach the earth's surface. This is 6,000 times as much as humans consume (15

terawatts – 10^12). The Arava has intense sunlight and receives 2150 kWh/m2

per year compared to 1700 kWh/m2 in Southern Europe and1300 kWh/m2 in south Germany .

Solar electric generation has the highest power density per unit area (global mean of 170 W/m2) among renewable energies.

Background

Background – Solar Energy

Different technologies can be used to convert solar energy into electrical power:

Thermal technologies : are considered appropriate only for large-scale installations

Photovoltaic (PV) technologies: are considered a reliable alternative to fossil fuel which can be implemented in a wide range of settings.

Background – Photovoltaics

Photovoltaic advantages

Little or no environmental pollution at the point of use.

Can work without tracking.

Systems are less complicated.

Requires minimum maintenance.

Scalable.

Background – Photovoltaics

Background – Photovoltaics

Example: SolFocus system:

7 kW array (unit for multi-MW systems)

Background – Photovoltaics

Example: FlatCon system:

PV technology can be implemented at different scales that range from:

Centralized PV power plants. Urban integrated. Integration of PV systems in individual

buildings (BIPV).

System scaling

System scaling: Centralized

Appropriate for large electricity demand and requires large area.

Concentration, tracking and special mounting are needed to reduce area, and improve efficiency and cost-effectiveness.

System scaling: Kibbutz/ urban integrated A good implementation of

micro-grids.

Potential for shading material for roofs, parking, open spaces.

Potentially reducing electrical transmission losses by having on-site-generation.

Benefit of the feed-in tariff.

System scaling: Building integrated

The potential utilization of built surfaces that already exist for other purposes.

concurrent savings of construction materials needed for supporting structures.

The substitution of cladding material with PV.

The possibility of recovering the thermal energy dissipated by the PV system and using it in different applications.

System scaling: Building integrated

Some countries subsidy green electricity by Feed-in tariff.

Responsibility on house owners for maintenance.

Hard to implement for large scale generation.

Suitable if used with energy conservation techniques.

Life-Cycle Energy Analysis (LCEA) accounts for both the input (Einput), or "embodied", energy required for production and maintenance of the system, and the output, or electrical energy generated by the system over a yearly cycle.

Energy Life Cycle

To be able to compete with fossil fuels, two important characteristics of PV systems are required: The energy supplied by the system over its

operational lifetime should be significantly greater than its embodied energy.

The net emissions of greenhouse gases from the PV system over its life-cycle should be significantly lower than the emissions from competing fossil fuel options.

Energy Life Cycle

The extent to which these requirements are fulfilled can be addressed by means of Life-Cycle Energy Analysis (LCEA) which accounts for: All energy inputs needed to produce the PV

module and the balance-of-system (BOS) components, as well as for the installation and removal processes, and compares it to

The savings due to electricity generation by the PV system over the years of its operation.

Energy Life Cycle

Prepare lists of PV technologies and installation types considered

Output Energy calculations:Get solar radiation data ,Wind speed &

temperature, Panel efficiency, Temperature coefficient, Shading

losses and GCR.Calculate the cell temperature.

Calculate the potential collectable energy, total yearly output & area per

unit output and GCR

Embodied energy calculations:Define the boundary of the system.

Data for different processes and materials.

Calculate total embodied energy of the system.

Calculate life cycle energy, EPBTFind out the electricity demand

Calculate electrical transmission losses.Examine different scenarios for different scales.

Calculate the potential output

Calculate life cycle energy and CO2 offset

The input energy (Einput) includes primary energy input for: Cell Materials. Capsulation Materials. The Balance-of-System: this includes all the

components except the PV module, including the support structure, foundations, the inverter, the tracker, electrical wiring, etc. Transportation.

Embodied Energy (Einput )

Embodied Energy

Life Cycle Energy Analysis - System boundaries

The system boundaries are defined in terms of the International Federation of Institutes for Advanced Study (IFIAS) scheme of orders as adopted by ISO 14040.

Level 2 covers at least 90% of direct energy inputs.

Embodied Energy – Manufacturing Processes

Silicon cell manufacturing processes

Metallurgical-grade silicon

Solar-grade silicon

Embodied Energy – Manufacturing Processes

Different stages for Thin Film module production

The potential collectable energy should be calculated based on the hourly global and direct beam radiation data.

The output energy for the photovoltaic cell depends on Collectable solar radiation Cell efficiency, which is a function of the cell’s

temperature.

Energy Output

where:

Icoll = the collectable solar radiation on the surface, W m-2.

Ib = the direct beam component of the solar radiation , W m-2.

θi = the incident angle.

Ki = the incident angle modifier for the direct beam component.

Id = the diffuse component of the solar radiation (Id=Ig-Ibcosθz), W m-2.

Kd = the incident angle modifier for the diffuse radiation. Calculated for θ=60° for diffuse radiation and θ=75° for ground reflected radiation.

β = the surface tilt angle.

Ig = the global radiation , W m-2.

ρg = the ground reflectivity.

Energy Output - Collectable Radiation

𝐼𝐼𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐼𝐼𝑏𝑏 cos 𝜃𝜃𝑖𝑖 𝐾𝐾𝑖𝑖 + 𝐼𝐼𝑑𝑑𝐾𝐾𝑑𝑑1+cos 𝛽𝛽

2+ 𝐼𝐼𝑔𝑔𝜌𝜌𝑔𝑔(1−cos 𝛽𝛽

2)

Energy Output –Cell Efficiency

Technology Nominal Efficiency Temperature coefficient %/°C

Multi-Si 14% -0.4%

Single-Si 19.3% -0.38%

a-Si 6% -0.25%

CdTe 10.76% -0.25%

CIS 12% -0.36%

Ribbon 13.2% -0.47%

SolFocus 25% -0.046%

FLATCON 26% -0.046%

The cell temperature was estimated by using the following model: Tm = back surface module temperature, °C.

Ta = ambient temperature, °C.E = solar irradiance on module, W m-2.Eo = reference solar irradiance, 1000 W m-2.WS = wind speed measured at standard 10m height, m s-1.T1 = empirical constant determining upper temperature limit at low wind speed, °C.T2 = empirical constant determining upper temperature limit at high wind speed, °C.b = empirical coefficient determining the rate

that module temperature drops as wind speed increases.ΔT = empirical constant determining the temperature difference between the cell temperature and the back surface module temperature, °C.

Energy Output –Cell Temperature

Type T1 (°C) T2 (°C) b ΔT (°C)

Glass/cell/glass 25.0 8.2 - 0.112 2

Glass/cell/Tedlar 19.6 11.6 - 0.223 3

Energy Output –Cell Temperature

Account for shading losses: Usually the calculations account for 15% losses in the flat-plate systems and 2.6% for concentrators due to mutual shading, assuming a 50% ground cover ratio (GCR) for flat-plate systems (40% for polar tracking) and a 12.7-17.5% GCR for concentrators.

Energy Output – Field vs. Cell

The Energy Payback Time:

The EPBT doesn’t give an indication of the net energy balance over the system’s life time in years (L). The Energy Return Factor (ERF) will express that, it is defined as:

Evaluation Metrics

The land needed per unit of output : This depends on the Ground Cover Ratio of the

system, the following values were assumed for the different installation types:

Evaluation Metrics

Installation type GCR

Stationary with tilt equals latitude 50%

Stationary with zero tilt ~100%

Single axis tracking 50%

Polar tracking 40%

SolFocus CPV 17.5%

FLATCON CPV 12.7%

The CO2 emissions offset: The amount of CO2 emissions from fossil fuel power

plants in Israel can be estimated based on the following table:

Methodology – Evaluation Metrics

coal gas heavy fuel oil total

Intensity of CO2 emissions (kg kWh-1) 0.951 0.600 0.894

Electricity percentage produced 75% 11% 14% 100%

Actual emissions per fuel type (kg kWh-1) 0.713 0.066 0.125 0.904

The CO2 emissions offset: In the calculations, a 20-year operational life time for

systems using thin film technology and 30 years for all the other systems is assumed, with a 1% yearly degradation.

CO2 emissions = Total Energy balance * potential CO2 emissions

Energy balance = Total Energy output – Embodied energy

Methodology – Evaluation Metrics

Evaluation Metrics

By multiplying the CO2 emissions offset by the GCR, the result will be the total CO2 offset per land area, with is a metric that accounts for all the different parameters of the PV systems in this study: embodied energy, yearly output, system life span and ground cover density.

Example – Energy Output

1373

1029

693

625

498

357

848

1149

0

200

400

600

800

1000

1200

1400

1600

Embo

died

Ene

rgy

[kW

h m

-2]

Technology

Example – Embodied Energy

Example – Evaluation Metrics

Single Si CIS

Example – Evaluation Metrics

Example – Evaluation Metrics

Example – Evaluation Metrics

N-S horizontal tracking Stationary with zero tilt

Example – Evaluation Metrics