Solar Air Conditioning / Solar Assisted Cooling :
"SAC Technologies"

The context: Air Conditioning sales to increase

Consumption for air-conditioning of buildings is strongly increasing. There are several reasons for this, such as the increased thermal loads in buildings caused by electrical devices, the increased expectations of comfort regarding the ambient climate, as well as the architectural trend to utilise glazed surfaces.

Global HVAC demand to rise more than 6% annually through 2014

Industry study from Bharat Book Bureau / Freedonia. (May 2010)

According to the Energy Information Administration (EIA), space-heating and cooling in the commercial sector accounts for 40% of total energy consumption. In the residential sector this number rises to almost 70%.
Conventional cooling systems utilise a compressor, which is usually electrically driven and hence energy-intensive to operate.

An alternative solution: Cooling with solar energy

Meeting these demands with solar energy is a particularly attractive option, due to the natural match between solar radiation availability and building cooling loads. On the basis of solar radiation and air-conditioning demand peaking at the same time during the day, it makes sense to resort to solar cooling and cover a considerable portion of the load with free energy derived from the sun.

Nowadays solar cooling technologies are neither as well known, nor as widespread as they should be. They have not yet enjoyed a significant level of penetration into the air conditioning market even if in recent years hundreds of solar cooling systems have been installed worldwide, most with a cooling capacity above 30 tons. (>100kW).

Domestic-sized units are also being developed. This is the current trend, where a number of manufactures are introducing low-power systems of less than 20kW capacity.

Air-conditioning with solar power, how it is done

Solar cooling installations generally consist of the components used in a conventional water heating system, i.e. solar collectors, storage tank, control unit, pipes and pumps plus a thermally driven cooling machine.

The solar collectors need to be a high-efficiency type, such as double-glazed, selective flat plate collectors or evacuated tube collectors.

In practice, solar collectors produce heat that is then fed into a thermally-driven chiller (cooling heat pumps) delivering refrigerant into a direct air-conditioning wall/ceiling unit. For large-scale, stationary applications, solar hot water is an excellent source of thermal energy to drive the chiller.

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Electricity consumption is minimal. In a typical design for room air conditioning, 1 kilowatt (kW) of electricity can produce up to 10 kW of cooling performance. This is why the heat-driven, solar-assisted chillers have an excellent potential in the space air-conditioning businesses.

Thermally driven heat pumps utilize sorption phenomenon. Sorption heat pumps can be operated with different working pairs (e.g. lithium bromide/water, zeolite/water, salt/ammonia or metal/hydrogen) and employ different operating principles, which may be absorption, adsorption or chemisorption (chemical reaction).

Solar AC Technologies: Open and Closed Refrigerant Systems

Cooling can be provided using either a closed or an open refrigerant system.

Closed systems use either absorption (liquid sorbent) or adsorption (solid sorbent) chillers to provide chilled water. Solar heat is used to regenerate the sorbent by driving off the refrigerant. Systems using absorption chillers are the most common.

Open systems supply cooled and dehumidified air using a desiccant cooling system and solar heat is used to remove water from the desiccant. Most common systems are desiccant cooling systems using a rotating dehumidification wheel with solid sorbent.

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Choosing a technology

Solar assisted air conditioning is a new and growing technology, compared to other fields of solar energy application.

Various technical solutions are possible, depending on the type and use of the building, on boundary conditions like existing technical infrastructure, and on other like climate conditions.

A simplified decision scheme for air conditioning technologies, applicable with solar thermal systems, is presented in the diagram on the left.

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Cooling using other heat sources

Heat pumps employed for solar cooling can use a number of different heat sources. Solar hot water panels are one solution but waste process heat or biogas are others. Whatever the application: domestic, office or industrial ; cooling can be carried out using natural, free sources of energy like the sun.

Environmental benefits of solar cooling

Energy consumption and carbon emissions decreased

Including ancillary components, a solar assisted cooling system can reduce electricity demand for room cooling by 60% compared with conventional cooling systems. As CO2 emissions go down accordingly, this make an important contribution to reducing our carbon footprint. Cooling, traditionally an energy-consuming process, becomes resource-saving.

Absence of harmful substances

To be faithful to the concept of sustainable solutions, we are aware that the materials and fluids used should be ecologically benign. The minerals used as adsorbate, silica gel or zeolite, are chemically inert. The refrigerant used in most cases is quite simply water. The materials used for the assembly are fully recyclable metallic alloys. Environmental responsibility goes beyond the useful life of an installation.

Noiseless systems

The lack of reciprocating and rotating parts results in a vibration-free and noiseless operation that can be important in specific applications.

Main advantages of solar air conditioning technologies:

reduction of summer peak loads in electricity demand
close seasonal and hourly coincidence of solar radiation and a building's cooling demand profile
absence of any ozone-depleting refrigerants
decreased primary energy consumption
decreased global warming impact
maximization of the exploitation of solar energy for heating, hot-water production and cooling by the same solar system