Solar air conditioning

Solar air conditioning

Days that have the greatest need for cooling are also the very same days that offer the maximum possible solar energy gain. Here we will look at different ways of using solar energy to cool air in a building.

It is possible to use PV to run the air conditioning, however there are better ways.

Standard cooling method

Air conditioning cools or warms air. The standard Air conditioning system is a refrigerator cooling air passing over it. 

Compress a vapour, it gets hot, heat is removed by ambient air, it liquefies. It passes through a small nozzle and expands. It cools in this process and the expansion chamber will cool any air passing over it.

In winter, the cycle is reversed. The heat is taken up from the cold air outside the building, (yes, cold air has heat in it),  and the heat from compression is released inside the building.

This is called a heat pump

Kerosene fridge - Solar thermal cooling

At first the old kerosene fridge with a flame producing ice  seems to defy logic. It has been around for over 100 years, so we can be sure it does work. Nowadays the coolers use hot water from rooftop solar thermal.

They work very simply.

Ammonia (vapour) dissolves in water.

Heat from a flame, or hot water from a solar rooftop heater, evaporates ammonia from the water. It is under pressure.

The ammonia passes through a nozzle where it expands and cools.

The ammonia vapour then dissolves in water again.

The process also works with water and Lithium Bromide.

This process is known as a closed system.

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Absoption cooling process

The diagram below shows a slightly more refined process where the heat given off by the ammonia dissolving in water is recovered and used to help evaporate the ammonia later on in the process.

NH3 + H2O  ---> NH4+  + OH-

Chromasun heats water with an array of Fresnell mirrors. The water needs to be above 88oC

The basic principle behind (solar-) thermal driven cooling is the thermo-chemical process of sorption: a liquid or gaseous substance is either attached to a solid, porous material (adsorption) or is taken in by a liquid or solid material (absorption).

Cooling with solar thermal heat using lithium bromide

This basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent. Water is the refrigerant and lithium bromide the absorbent. These fluids are separated and recombined in the absorption cycle. In the absorption cycle the low-pressure refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller. Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases. The added heat causes the refrigerant to desorb from the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent, in the generator passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapors returning from the evaporator so the cycle can be repeated.

Source

Desiccant - open cooling system

Air can pass over a desiccant such as silica gel, or calcium chloride solution. This absorbs water and dries the air. The air can then be cooled by evaporating water into it.

The desiccant is then dried by solar heat. The advantage of calcium chloride solution is that it can be pumped to the roof by a solar powered pump

DEVap air conditioner

Here’s how the desiccant-enhanced evaporative, or DEVap, air conditioner works.  A polymer membrane coated with both a teflon-like substance that repels liquid water and a desiccant divides the air flowing through the system into two streams.  The membrane has pores about 1 micrometer to 3 micrometers in diameter; these are large enough for water vapor to pass through but too small for the desiccant to sneak across.  The desiccant draws moisture from the airstream, leaving dry but warm air.  Indirect evaporative cooling takes place in a secondary chamber, chilling the other half of the divided airstream.  As the air in the second chamber grows cooler and wetter it cools the dividing membrane, which in turn cools the first airstream, and out of the machine comes cool, dry air.  The process uses up to 90 percent less energy, depending upon the humidity of the air that goes into the system at the start (Source: NREL).  

NREL’s liquid desiccant takes the form of a 44% salt by volume solution of lithium chloride or calcium chloride (aka road salt).  The corrosiveness of the salt “requires that metal be eliminated from the hardware. 

Evaporative cooling

A more simple version can be used in dry areas with sufficient water. An evaporative cooler simply evaporates water into the air. This cools the air leaving less to be done by sweat evaporating on your skin.

There are some systems that divide the incoming air into two streams. One is cooled by contact with water. This cool air then cools the other stream leaving it cool and dry.

Solar powered ventilation

Solar PV powered fans can circulate air.

This can be improved by passing the air through a heat exchanger to help cool or warm the incoming air.

A chimney can enhance cooling by drawing up hot air without the need for a fan.