Solar PV in hot climate zones
Design considerations and technological options for solar PV systems in hot climatic zones
Background:
Since 2009, GIZ Energising Development Ethiopia (EnDev ET) has been installing stand-alone solar PV systems of different sizes (between 300 and 2.400Wp) for social institutions all over the country.
Most of the 300+ installed EnDev ET solar systems are situated in the highlands or sub-tropic zones that feature moderately-warm temperatures that are ideal for the uninterrupted operation of these systems. In the north-western (Afar) and south-western (Somali) regions of the country, however, scorching sun meets hot desert climate, with average annual temperatures around 30°C (86°F) and peak temperatures reaching easily above 40°C (104°F). These circumstances inspired the redaction of the present working paper that attempts to compile different responses regarding system design and technical components to extreme air temperatures.
This contribution to Energypedia is explicitely open for suggestions, comments and additions from any colleague who has experience in setting up solar PV systems in hot climatic zones.
To start with, the article outlines the three main components, i.e. solar PV panels, inverters and batteries, to receive special attention when being used in hot climatic zones:
For those without a technical background in solar energy, it may seem counter-intuitive that solar PV panels face significant challenges when being used in areas with high levels of sun irradiation and increased air temperatures. Usually, PV modules are tested at a temperature of 25°C (77°F), but if the temperature of the solar panels increases, the voltage output decreases in direct correlation and linearly. This has significant effects on the output of solar panels - even in areas with medium levels of solar irradiation (around 2000 kWh/m²) where temperatures within the solar module can reach up to 100°C (212°F). This reduction of the peak output of the solar panel for each 1°C is expressed with the Pmpp-coefficient that should be indicated at every solar PV panel datasheet. As example, a solar panel whose peak output is tested at 25°C and has a Pmpp co-efficient of 0.4% will lose 25% of its output once the internal temperature reaches 100°C![1]
Moreover, high temperatures can foster the “Potential-Induced Degradation” (PDI) that is typical for most crystalline solar modules and results in so-called leakage currents that flow through the embedded parts and the backside of the module. These leakage currents foster electrochemical corrosion within the module and can significantly reduce its performance too.[2]
Given the effects of high temperatures on the solar module output, there are ways to mitigate this effect through the right installation of the panels[3]:
- Panels should be installed on the roof or another carrier with enough of space for airflow in-between to cool down the modules.
- Heat absorption may also be reduced if the panels are installed on and next to light-coloured material.
Furthermore, there are solar panel manufacturers who are specialized on particular heat-resistant panels. The manufacturer J.v.G Thoma (www.jvgthoma.de) and its distributing agent Jurawatt (https://www.jurawatt.de) for example provide solar panels that are tailored to desert and tropical areas and feature the following characteristics[4]:
- Maximum (sustained) operating temperature of 125℃ (as opposed to 85℃ that standard PV panels allow) and resistance to peak temperatures up to 145℃.
- Free of PID-related losses.
- Hybrid version available that includes a thermal collector for absorbing parts of the heat.
It is assumed that the costs of these special panels are higher than those of standard panels. The supplier, however, did not the pricing information on request to date.
Inverters of reputed manufacturers (Outback, SMA, Victron etc.) can operate within temperatures up to 60-65℃. Their continuous AC-output, however, decreases with rising temperatures. Starting from the rated continuous AC-output at 25°C (77°F), the continuous output of inverters decreases for example at 40°C (104°F) by 6-15%, depending on the model.[5]
Therefore, over-dimensioning of inverters is a way to mitigate performance losses at high air temperatures. The indications on the datasheets of inverter models are to be referred to for calculating how much the over dimensioning of the inverter’s capacity would be required to provide sufficient AC-output at consistently elevated air temperatures.
Furthermore, the technical room where inverter and the battery system are usually stored, should have enough air inlets for ensuring the maximum of airflow to cool down the system. Depending on the size of the solar PV system, a ventilating system may be considered to support the airflow in the technical room and thus keep temperatures in check.
Lead-acid batteries are still by default used in most larger solar PV systems across the world. Apart from their highly toxic content consisting mostly of lead, diverse lead compounds and acid, their lifetime is proven to decrease rapidly with increasing temperatures.
The nowadays commonly used sealed, maintenance free gel-batteries (VLRA-type) can easily reach lifetimes of 10+ years under temperatures at 25°C (77°F).[6] However, rising temperatures have significant effects on the electrochemical activities within the lead-acid battery, increasing on the one hand its capacity, but decreasing on the other hand the voltage output due to higher resistance and band gap that are caused within the battery with rising temperatures.[7] Furthermore, increased cell temperatures lead to higher electrochemical activities that foster corrosion within the battery, hence diminishing the battery capacity and its total lifetime in the long run[8].
The prospected decrease in lifetime for different battery types illustrated below[9]:
|
Average temperature |
AGM Deep Cycle |
Gel Deep Cycle |
Gel Long Life |
|
Years |
Years |
Years | |
|
20°C / 68°F |
7 – 10 |
12 |
20 |
|
30°C / 86°F |
4 |
6 |
10 |
|
40°C / 104°F |
2 |
3 |
5 |
According to durability test results, after roughly every 8°C - 10°C temperature increase, the lifetime of a sealed lead-acid battery is reduced by 50%. And once the corrosion has progressed because of steady exposure to hot temperatures, the lost capacity cannot be restored anymore.[10]
One way to at least slow down the rapid decrease of lifetime of sealed lead-acid batteries is to combine it with a charge controller that features a temperature-adjusted regulation of its charging voltage and float voltage. Reputed producers offer charge controllers that reduce the for example the float voltage with every 1°C temperature increase by 30mV, summing up to 0.03V less float voltage for a 10°C temperature increase[11]. These adjustments may extend the battery lifetime by 15% given the still persisting corrosion caused by increased temperatures.[12]
The below table illustrates the voltage adjustments for charging and maintaining the stationary charge of lead-acid batteries at different temperature levels:
|
Battery status |
-40°C (-40°F) |
-20°C (-4°F) |
0°C (32°F) |
25°C (77°F) |
40°C (104°F) |
|---|---|---|---|---|---|
|
Voltage limit |
2.85V/cell |
2.70V/cell |
2.55V/cell |
2.45V/cell |
2.35V/cell |
|
Float voltage |
2.55V/cell |
2.45V/cell |
2.35V/cell |
2.30V/cell |
2.25V/cell |
Source: Battery University (see footnote no. 12)
Alternatively, solar PV installers might resort to Lithium Iron Phosphate (LiFePO) batteries. This type of battery is increasingly replacing lead-acid batteries for solar PV applications, featuring a higher number of life cycles, deep discharge abilities without memory effect and – as opposed to Lithium-Ion-batteries – a highly reduced affinity to thermal runaways in spite of its high energy density[13].
Moreover, the electrochemical composition of this battery type proved to be considerably more resistant to high temperatures than lead-acid batteries. The number of life-cycles as well as the storage capacity of LiFePO-batteries is only marginally affected by temperatures up to 45°C (115°F). For example, over a storage time of 150 days and at a temperature of 45°C, a fully charged LiFePo-battery only loses 25% of its charge[14].
Charging LiFePo-battery at temperatures of 45°C (115°F) and discharging at temperatures of 65°C (149°F) is feasible[15], although the degradation of battery increases at charging temperatures of 30°C (86°F) and higher[16].
Although LiFePo feature a significantly stronger resistance towards high temperatures compared to lead-acid batteries, their weak side, however, is in lower temperatures, i.e. 0°C and below, as this significantly increases the resistance within the cells. Therefore, low temperatures reduce LiFePo battery’s charging ability and its capacity[17][18].
This may become a disadvantage when being used in areas that feature temperature extremes from hot to cold, what may be the case in some desert zones.
The different electrochemical reactions of LiFePO-batteries to temperatures as compared to lead-acid batteries will also require an adjustment of the temperature-based voltage compensation of a charge controller, if not a LiFePO-specific charge controller is in use.
Conclusion
The initial findings of this working paper allow for the conclusion on a couple of technological and installation-related adjustments in order to cope with high air temperatures around solar PV systems in hot climatic zones.
While the installed solar PV panel capacity may simply be over-dimensioned to compensate for the performance loss due to high temperatures, the overheating of the panels can also be reduced by following a few simple mounting recommendations for reducing heat absorption and increasing airflow. For those projects that wish to use specialized solar PV panels that are adapted to the challenging temperatures in desert zones, one exemplary producer and supplier has been presented. It is to be assumed that the costs of these panels are higher than for ordinary ones.
Regarding the inverter performance, an over-dimensioning of its capacity seems to be again a rather simple way to cope with high temperatures. However, the set-up of the technical room regarding the position of the windows, availability of sufficient airflow and the possible installation of extra ventilation systems may need to be considered here in addition.
Commonly used lead-acid batteries proved to be one of the major weak points of a solar PV installation in hot climate zones, as their lifetime significantly decreases with rising temperatures. Lithium-Iron-Phosphate batteries may be an appropriate alternative, as they are more resilient to hot temperatures. However, Lithium-Iron-Phosphate batteries require adjustments in the automatic charge controlling and should be used only in areas that do not feature drastic temperature falls to 0°C and lower.
[1] https://www.civicsolar.com/support/installer/articles/how-does-heat-affect-solar-panel-efficiencies
[2] https://www.jvg-thoma.de/en/photovoltaic-technology/desert-technology/high-temperature-resistance/
[3] https://www.civicsolar.com/support/installer/articles/how-does-heat-affect-solar-panel-efficiencies
[5] Datasheets of different models under: https://www.victronenergy.com http://www.outbackpower.com/products/inverter-charger https://www.sma.de/en/products/battery-inverters
[14] Hato et al.: Degradation Predictions of Lithium Iron Phosphate Battery, World Electric Vehicle Journal, Vol. 07, 2015, [http://www.mdpi.com/2032-6653/7/1/25/pdf
www.mdpi.com/2032-6653/7/1/25/pdf]
