Compared to solar photovoltaics some might find solar thermal technology to be less attractive but in reality the opposite is true in many cases. Why do I say that? Because you can not only use solar thermal for heating but also for cooling.
Yes, we can use solar thermal for dehumidification and air conditioning. Other than that we can also now use it for energy storage in a very unique way. Let’s begin by laying down the basic advantage of solar thermal technology, which is its high energy conversion.
Solar air conditioning can either be normal AC powered by solar PV panels and an inverter, or be powered by thermal heating panels. The two types of pure solar AC are the desiccant cooling system and vapour absorption based cooling systems. Thermal water heating panels have a conversion efficiency of between 80 and 90%.
In evacuated tube collectors, for instance, we can convert 81% of the solar energy into thermal energy. When compared to a typical PV panel which has an efficiency of 19% we can see that over four times more energy is made available for utilization by solar thermal for the same amount of sunshine.
Is solar air conditioning worth it?
One might argue that electricity is a better form of energy compared to heat. However, if that electricity is ultimately going into providing heating or cooling for the household, then it doesn’t make sense to use it for these applications.
Solar thermal technology over the last 10 years has further improved. The development of integrated compound parabolic collectors in evacuated tubes and the use of nano fluids has increased the efficiency of solar thermal systems to up to 90%. We have seen the emergence of commercial products like solar kettles and solar barbecue grills in the last decade.
We already know about solar water heating. Once the water is heated through the solar collectors it can be used for cleaning, or heating indoor space, or even swimming pools for that matter. What about space cooling and air conditioning?
Well, this is where the development in the last 10 years has been most exciting. There are two systems that we will be mentioning. First is the desiccant cooling system and the other is the vapour absorption system. Both can use sun’s heat directly as an energy source to run.
What is solar desiccant cooling system?
Let’s first look at the desiccant cooling systems. The idea here is to remove the moisture from the air. Many of you would realize that it is not often the temperature of the air that is the cause of discomfort but it is the humidity level. The comfortable range of humidity for an enclosed space is between 30 to 60%, while it’s recommended to be 50 to 55%.
In many places around the world humidity levels remain above 80% for most part of the year. In cold climate countries higher humidity levels not only cause mould to build up but also creates health problems. In warmer regions high humidity makes us feel stiflingly hot.
The solar desiccant cooling system that is used to remove the moisture from the indoor space – the way it’s done is by the use of a desiccant wheel. When room air is blown through the wheel, which is made of ceramic or contains silica gel, it soaks up the moisture from the air.
This dry air is let back into the house but what do we do with the moisture now trapped in the desiccant wheel. Well, we blast it with hot air. The hot air is created in a separate loop. We take the outside air and pass it through an air-to-air heat exchanger.
The heat exchanger is heated by a solar thermal collector. The hot air coming out of the heat exchanger is pushed through the section of the desiccant wheel which is rich with moisture. The heat in the air helps to release the moisture into it. This moist air is released outside.
It is a very simple system which requires a fraction of the energy compared to a compressor based dehumidifier. Note that in many cases the use of dehumidifier eliminates the need of using an air conditioner completely. In a humid environment, if an air conditioner is used, then nearly half of its energy can go towards removing the moisture from the air.
This shows the impact the humidifiers can have on our comfort requirements. Desiccant dehumidifiers are widely used in the industry. They can bring down the humidity levels to as low as 30% They have been slowly making their way in domestic sector but they use electricity to heat the air through a heating element.
The need is to replace this with a solar air heater. Currently, there are only a handful of companies that provide solar based desiccant dehumidifiers but this can be easily made at home and you’ll find Youtube videos that help you with this DIY project.
How does solar air conditioning work?
Another thing that can be made is what I would like to call a pure solar air conditioner. I’m not talking here about the normal air conditioner that uses a vapour compression cycle. Most people think that powering this AC unit with solar PV panels is what solar air conditioners are.
No, I’m talking about a vapour absorption cycle that has no compressor at all and uses heat as an energy source. This system has been used for refrigeration and air conditioning for a long time, it’s just relatively unknown.
Until the 1960s most of the air conditioners were based on a vapour absorption cycle and they burnt gas as the source of heat. It was later that more compact vapour compression based units arrived and replaced them when electricity became cheaper.
What is vapour absorption system?
We still see vapour absorption based cooling systems being used in the industry as they can utilize waste heat. The term ‘absorption chiller‘ is used for these systems. People have designed solar powered absorption chillers. They are very robust and can be placed outside.
The only moving parts they have are two small pumps. One that circulates the coolant inside the refrigeration cycle loop and the other circulates hot water from the solar evacuated tube collectors to the section called the generator.
Bear in mind that in some of the modern evacuated tube collectors temperatures of over 200 degrees can be achieved on a bright sunny day. The higher the heat supply temperature in the generator section of the vapour absorption chiller, the more efficiently it works.
The only downside they have is the size. For the same cooling capacity, absorption chiller based systems can be more than three times larger but the advantage is they don’t take up your inside space. Above all, they cost peanuts to run.
Yes, instead of looking at 1800 watts to run a vapour compression based normal split unit air conditioner, you’re looking at only 200 watts to run a solar vapour absorption system. The electricity is used just to run the small pumps to circulate the refrigerant and the heating fluid from the solar collector.
DIY off grid solar air conditioner.
The main energy needed to run the cycle comes from solar heat. The question now is can you make a small size vapour absorption AC at home? Yes, absolutely you can. The easiest way to do it would be to use a second-hand three-way motorhome or an rv fridge. Some of you might know that these refrigerators can burn propane as a heating source.
They can also use DC battery or grid electricity, hence the name three-way fridge. You will simply have to take the generator section of the fridge and connect it to a hot fluid tank that is heated by the solar evacuated tube collectors.
You will have to cut a large hole into the front pitch door to accommodate an inline ducted fan. A simple exhaust fan or a ventilator fan can also be used. In addition to that holes will have to be drilled for air to be sucked by the fridge cabin.
This system will be able to cool a very tiny space. For larger rooms multiple large three-way fridges can be used. Interestingly you’ll find a lot of these three-way fridges in your local scrap yard that are sold for very little, so this could be a nice DIY project for the summer.
One more exciting development in the world of solar thermal technology is the heat battery. A Scottish company called Sun Amp has developed these batteries which can store large quantities of heat. This heat can then be released using a trigger system.
What is a heat battery?
Heat batteries can be coupled with PV electricity or solar water heaters to store spare energy. Bear in mind that most of our energy usage is in the form of heat. We have made rapid strides in electricity storage but not the same can be said for heat storage.
Very rarely we find that large water tanks have been utilized for inter-seasonal heat storage. Through these super compact heat batteries a lot of renewable energy can be stored safely and securely. Siemens is also looking at providing heat batteries at an industrial scale using molten silicon separately.
Molten salt based heat storage is used in concentrated solar thermal plants that have mushroomed around the world. The energy storage allows the solar plants to provide round-the-clock base load during sunny days. At present there are concentrated thermal power plants with 6.4 gigawatts of nameplate capacity, 5.5 gigawatts of which were installed in the last 10 years.
From 2010 to 2012 there was a lot of interest in the development of solar dish sterling engines. These reached energy conversion efficiency of over 28% This interest has subsided since because of the emergence of low cost PV.
If the ultimate utilization of energy is in the form of heat, then the most efficient way is to use solar thermal rather than using solar PV and converting electricity into heat.
Can solar panels power air conditioning?
Solar panels can power air conditioning using a batteries and an inverter for converting the voltage from DC to AC for the AC compressor. Several solar panels are required and substantial battery capacity in order to accommodate the surge current associated when large AC compressor motors start.
How does solar thermal air conditioning work?
Pure solar thermal air conditioning uses a vapour absorption cycle that has no compressor at all and uses heat as an energy source. This system has been used for refrigeration and air conditioning for a long time but isrelatively unknown.