Solar Air Conditioning – How Solar Air Conditioning Works?

Before talking about how solar air conditioning and solar cooling works, one must have some basic information about passive solar water heating. The passive solar water heating basically uses a pump for circulating heated liquid in solar collectors. The heated liquid is then used for solar air conditioning and cooling using Lithium Bromide (Li-Br) absorption chillers.

To make it easy on you to understand how solar air conditioning and cooling works, an illustrative diagram is shown below to explain the process for passive solar water heating. An explanation will follow on how to get solar air conditioning results. i.e., Absorption chillers use heat for cooling.

Passive Solar Water Heating - Forced Circulation

The diagram above shows a passive (forced circulation) system that uses a number of solar collector plates (or can be vacuum tubes) to harvest free energy from sun rays.

A liquid absorbent (Glycol or water) transfers heat from the solar thermal collector panels. A pump forces
the hot liquid to pass through insulated pipes to a heat exchanger or coil inside the insulated water storage tanks in the building. When the hot water reaches the heat exchanger coil in the water storage tank, it transfers its heat energy from the hot water (shown in red) to the cold water inside the water storage tank and it continues to flow back to the solar panels for the process to continue again.

Note that the liquid in the solar collector panels will not mix with the cold water of the water storage tank, however; hot water transfers its energy to the cold water inside the storage water tank and by doing so it becomes cold (Shown in blue).

The circulating pump is driven by a programmed controller to keep circulating the liquid from the solar thermal panels to the heat exchanger as long as the temperature in the in the solar thermal panel is greater than the desired temperature in the storage water tank.

Solar Water Heating With System Controller

Conventional Cooling Chillers V.S. Absorption Chillers for Solar Air conditioning:

Chillers in conventional Air-conditioners use mechanical energy such as a motor or engine to pressurize gas or refrigerant. By doing so, conventional chillers consume a lot of electric power to mechanically pressurize the refrigerant gas. In a typical building it may consume as much as 60% of the total building power.

To the contrary, an absorption chiller which is the oldest form of cooling does not use an electric compressor to mechanically pressurize the refrigerant gas. Instead, the absorption chiller uses a heat source from a large set of solar thermal collector and consumes less than 10% of the total electric power required by the same building.

Components of an Absorption Chiller:

An absorption chiller consists of a generator, a condenser, an evaporator and an absorber to replace the motor or engine driven vapor compressor that is used in a conventional air-condition. An explanatory diagram is shown below:

Components of an Absorption Chiller:

How does the Water and Lithium Bromide Absorption Chiller Works?

There are four compartments in an absorption chiller. All compartments are all at low pressure. A heat medium from solar thermal panels or any other heat source such as natural gas or waste heat source enters the generator chamber at temperatures above 70 Deg. C and heats the mixture of Li-Br that is diluted in water. Water evaporates and flows to the condenser chamber leaving behind concentrated Li-Br liquid in the generator chamber.

The condenser chamber is cooled with an external water cooling tower. When water vapor reaches the condenser chamber it condenses and becomes liquid water. As the water starts to flow from the condenser to the evaporator chamber, it gets sprayed on the chilled water pipe of the Fan Coil Unit that is used for Air-conditioning.

Simultaneously, the hot and concentrated Li-Br that flows from the generator chamber gets cooled when it passes through the heat exchanger. When the cooled concentrated Li-Br is sprayed on the cold water coil in the absorber chamber it recombines with the condensed water that entered from the condenser.

When water and the concentrated Li-Br get recombined, the chemical process has a property of absorbing heat and the only available source to extract heat is from the chilled water pipes. Therefore, the chilled water pipes give away heat (near 6 deg C) and become cold. The Lithium Bromide absorption chiller cycle can be seen in the diagram below.

Lithium Bromide Absorption Chiller Cycle

Advantages of using Absorption Chillers for Solar Air conditioning:

• Use of free heat source, the sun.
• Hot and/or cold water output during day time.
• Eliminates electricity use below 10% instead of 60%
• Environmentally friendly with zero emission.
• Low noise level near 50dB A.

Solar air conditioning that uses passive solar water heating panels can sometimes be the alternative solution and may fit when a source medium of free or low-cost heat such as solar thermal panels is available. Regretfully, as with all solar and renewable energy products, the initial cost and space requirements for solar air conditioning and solar cooling is extremely high.