Optimizing the efficiency of solar energy during the July heatwave

As we gear up for yet another blistering summer soon, let's consider the solar panels perched up on the rooftops, battling the heat. People often misunderstand photovoltaic rooftop solar to flourish in high heat conditions similar to thermal solar systems; in reality, however, the situation is quite the contrary.This article provides necessary design and procurement techniques that, once implemented, will ensure the stability of your solar system during the expected July heatwave and help you achieve peak energy production.

What’s The Big Deal With Heat And Solar?

Photovoltaic (PV) solar panels have the ability to convert sunlight into electricity. However, their productivity reduces as the temperature increases. Unlike solar thermal hot water panels, solar PVs function better in relatively cooler conditions. Therefore, employing a good design is a key factor to counteract the impact of heat on power output.

Under the Standard Test Conditions (STC), a solar panel's power is defined at a cell temperature of 25ºC. When deployed in real-world conditions, the panel's performance will typically decline due to the increase in temperature. The fraction by which it declines is known as the temperature coefficient (Pmax), which is measured at Nominal Operating Cell Temperature (NOCT). NOCT, among other parameters, specifies an air temperature of 20ºC during laboratory testing.

Taking into account that the average NOCT is roughly 45ºC, it's reasonable to conclude that in real-world conditions, any day with air temperature exceeding 20ºC will negatively affect the efficiency of a solar panel due to heat. Although this impact might be negligible on a moderate day, a few degrees increase in the temperature could substantially alter the situation.

From California's data last year, it's evident that high temperatures are a significant challenge in optimizing solar energy production. Half of the days saw peak temperatures over 20 degrees and for more than a quarter, temperatures rose to around 30 degrees. Such conditions lead to notable temperature-related efficiency losses in solar panels, a challenge that needs resolving.

Absolutely, not all days yield the same solar energy, especially those with higher temperatures that often have more peak sunlight hours. It's indeed a double-edged sword. On the one hand, there's increased solar irradiance, and on the other, a challenge in the form of decreased panel efficiency due to high temperatures.

Reflecting on the number of sunlight hours, we see that most occur when temperatures rise over 20 degrees, with over a third even on extremely hot days above 30 degrees. This data underscores the importance of considering temperature variations when evaluating solar panel effectiveness in different weather conditions.

Does The Temperature Coefficient Matter?

When exploring solar energy investments, it's beneficial to choose a solar panel designed to perform optimally in hot conditions. In reviewing the datasheet, aim for the temperature coefficient (Pmax) with the smallest negative value near to zero, implying a lower power reduction. Therefore, a parameter like -0.37%/°C is more advantageous than -0.40%/°C.

If we look at the solar panels with the highest and lowest temperature performances, there's only a 0.31% variance for each rise of 1°C over the standard 25°C (for instance, top at -0.21%/°C versus the lowest at -0.52%/°C). Translating this to real-world conditions, this equates to a maximum potential efficiency drop (from highest to lowest) of around 3% on a 30° day and 6% on an intense 40° day.

Indeed, the efficiency loss doesn't occur uniformly throughout the day, as it constantly fluctuates with the changing temperatures. Due to this dynamic nature, it becomes practically challenging to calculate the specific amount of energy or dollars saved between different solar panels in terms of temperature-related efficiency loss.

The importance of the temperature coefficient can be a subject of debate. While some individuals might perceive it as insignificant, it's crucial to remember that a solar panel with a low temperature coefficient often excels in other specifications too. Therefore, it could be worthwhile to take it into account during the selection process for the associated overall quality.

Air Conditioning And Solar

As we endure one heatwave after another, concerns about the rising expenses of operating an air conditioner are completely justified. It would indeed be sensible to meticulously plan out the sizing and design of a new rooftop solar system, which can help to offset these costs significantly. The better optimized your solar setup is, the more you'll save in the long run.

First Things First

It's imperative to underline the importance of proper insulation for your building envelope before considering the solar option. The only truly free energy is the one you don't consume, so it's essential to, at the very least, confirm the adequacy of your roof insulation and rectify any gaps in doorways or windows.

Presuming that your insulation is taken care of, let's then suppose you would want your new rooftop solar system to offset energy expenses of a mid-sized air conditioner operating in the peak hours of the day during the most intense heat of the year. We would later discuss the winter scenario.

Tailoring Solar Power for Air Conditioner Load

Here's a straightforward 4-step approach to dimensioning a grid-connected rooftop solar system to accommodate your air conditioner's load. This strategy might not fit if you have different priorities or for an off-grid system. Also, it should be used alongside guidance from your installer.

MS-450W Solar Panel Parameters

1 Determine Air Conditioner Power:

For this illustration, let's take a 7 kW (output) split AC system having a peak load of 2.5 kW. With a bit of complex calculation, we'll consider an average running load of 2 kW.

2 Determine Daily Power Usage：

To figure out the daily energy usage during high-heat conditions, just multiply the average power with the anticipated running hours during the hottest day.
2 kW (average power) x 16 hours (daily operating time) equals 32 kWh (daily energy usage).

3 Determine Power Needs for Warmest Month:

Let's make more assumptions: Suppose you live in California, where each kW (kilowatt) of rooftop solar generates an average of roughly 4.4 kWh (kilowatt-hours) of daily energy. But for our calculations, we're only focusing on your energy needs during high-heat conditions. Hence, you need to find out the daily kWh energy production per installed kW of solar during your location's hottest month. Therefore, you should plan on installing 5.8 kilowatts of solar power daily.

4 Determine Required Solar Panel Power:

Split the daily energy usage of your air conditioning unit by the daily energy production per kilowatt of solar power.32 kW (daily energy usage) / 5.8 kWh (daily energy production per kW of solar power) = 5.5 kW (necessary rooftop solar capacity).The above does not take into account any other house loads. It's merely a method to calculate the solar energy required to power an air conditioner during the hottest period of the year for a grid-connected system. There are numerous other factors to consider when designing a rooftop solar array for a household.

Direction of Solar Panel Placement

Orienting your solar panels towards the west can be beneficial, especially during heatwave conditions, as it maximizes solar energy production during peak afternoon hours. This coincides with the highest usage time for your air conditioner and likely other appliances within your household. Therefore, this arrangement can enhance the efficiency of energy self-consumption.

Solar Panels Favor a Cool Wind

Just as we enjoy a refreshing breeze on a hot summer day, so too do our solar panels. Research has proven that wind has a cooling effect on PV cells, enhancing the solar panels' electrical output. Ideally, this breeze should arrive before 2 pm, ideally from the direction the panels are facing.

Indeed, in California, we're fortunate. Those dwelling within 5 or 10 km of the coast benefit from the consistent southwest sea breeze. It usually kicks in around midday on most summer days, except the most sweltering ones that exceed 30 degrees. This wind direction is particularly beneficial to solar panels with a western orientation, enhancing their performance.

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