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Solar Powered A/C

An experiment was carried out to see if our house could be cooled using solar power.

Background

In the not too distant future, everything must be powered by renewable energy. Our current renewable energy production isn't sufficient, so we will have to reduce the value of everything, consume less, and find ways to collect more renewable energy.

Air conditioning uses a lot of energy. Is it something we could do without? No, we couldn't. Climate change leads to rising temperatures and more severe and frequent heat waves. We're already seeing dangerous combinations of temperature and humidity. Air conditioning will not be considered luxury in the future. It will be a necessity.

To make matters worse, air conditioning is still often powered in a way that results in more greenhouse gases being emitted to the atmosphere. This in turn leads to more heat, more air conditioning units being installed and the existing ones running for longer periods of time. We are accelerating climate change while attempting to alleviate its effects.

We therefore need to find ways to power air conditioning using renewable energy.

Luckily supply and demand of energy couldn't match much better than they do with air conditioning and solar panels. We had recently built a small solar power system at our house. It was mainly used to power small continuous loads, such as freezers, a few computers, and a few intermittent loads such as charging phones, laptops and power tools. The system is not connected to grid, so it only provides energy we have actually collected.

Our micro power plant was designed to power loads consuming about 2kWh per day. The parts are matched properly for this, so we haven't had to worry about running out of power. On the contrary, we have occasionally worried where to dump the excess energy when the sun is shining and the batteries are already full.

One solution has been to boil water for doing the dishes and taking showers. While useful, this has a few drawbacks. Firstly we don't want to create extra heat in the house during hot days. Secondly, we only have devices that can do resistive heating. This makes the efficiency abysmally low, since we're essentially using just the waste heat. We could achieve just heating on such days even by having a black water tank outside. A much better option would be something where clean electricity gives an edge, such as a heat pump, like in air conditioning..

Numbers

We have a fairly old A/C unit with around 2.4kW of cooling capacity. During a hot summer day it runs for about 8 hours per day to keep the house cool.

Luckily cooling power isn't the actual power requirement of the device. Heat pumps have an efficiency rating based on how much heat energy they can move per watt of consumed power. Our A/C unit has an efficiency ratio of about 5, so the actual power requirement in a typical temperature- and humidity setting is roughly 2400W / 5 = 480W.

The power needs to be 230V AC, so we need an inverter. This brings in some extra cost and conversion losses, but on the plus side we get to use also other AC devices and can use smaller wires to carry the power. Let's say the total conversion and transmission losses amount to 8% and we use an inverter which needs 10W for itself. We will thus need roughly (480W * 8h) / 0.92 + 10W*24h= 4.3kWh of excess production on sunny days to be able to power the A/C.

This sounds quite doable. Would something like this make a difference if done in wider scale? Finland has about 2,600,000 households [link]. Suppose 2,000,000 will have A/C in the future with similar units. Suppose we have a hot month and air conditioning is used every day. That would mean 2,000,000 * 30 * 4.3kWh = 2.6TWh of energy. That should sound scary! The total nuclear power based electricity production in Finland was just 21.881TWh per year in 2018 [link]. That would mean 1.8TWh per month. All of the current nuclear reactors combined wouldn't produce enough electricity to cool most houses during a hypothetical future heat wave. This may sound far fetched here in Finland, but one doesn't need to look far south to find countries already spending over half of their electricity on air conditioning [link].

Solution

The plan was to upgrade our micro solar power plant so that the amount of excess production during hot days would match our need for air conditioning, so that our A/C can run off the sun.

We initially had two 310W solar panels on the roof. We got two more and connected them to 2s 2p configuration now totaling 1.2kW. This should allow us to collect some more energy later in the year, and we should be getting enough on sunny days for both A/C and the normal loads. On a cold sunny day we're now getting up to 1.4kW, but on hot days it's closer to 1.0kW. Solar panels don't like heat either.

We already had an oversized 100V/50A charge controller. The battery on the other hand was just 12V. This meant the most we could have taken off the panels would have been 12V * 50A = 600W - barely enough for A/C. We decided to upgrade the battery to 24V, which halves the currents and allows us to get more use out of the panels on good weather. The upgrade meant getting 4 new LiFePO4 cells, swapping the BMS to a 8s one and reprogramming the charge controller accordingly.

Then we just needed to convert the DC to AC. We decided to go with a 2kW inverter, which is also able to handle also our small oven and electric chainsaw on occasion.

Results

Works like a charm!

We're getting roughly the expected 4kWh/day as expected. The A/C has been on as needed and we've had a stable 24°C inside since the system was set up. Temperatures outside have so far peaked at 30°C.

It is definitely possible to power air conditioning with a few solar panels.

#solar #cooling