How Solar Cooling Works

Design

Solar Cooling systems are comprised of several key components/stages:

Firstly, the Sun's heat is obtained through the use of solar collectors, before being converted into cold using a “sorption” cooling process. The resulting cold must then be delivered to the application using a heat transfer medium which is typically chilled water or dry cool air. Thermal storage of the collected solar heat (or that of the produced cold) is often used to extend the operational hours of the solar cooling system. A number of alternative component technology options are available for each of these steps.

Component options for each stage include (note: not all listed options are compatible):

Each of the variants have advantages and disadvantages, and no one technology combination has yet been shown to be the optimum.  For example, low cost collector technologies tend to only provide a low temperature source of heat which often requires a more costly or less efficient cooling process.  Furthermore, application requirements or customer preferences may determine a delivery method which has a resultant impact on the selection of the cooling process and solar collectors.

Heat Collection

Collector plant selection is dependent on the required operating temperature of the chosen cooling process.

Air collectors are relatively low-temperature collectors used to directly heat air.  Air collectors are commercially available in Australia and have been particularly used for building space heating applications in cooler climates.  Air collectors have substantial cost reduction potential through building fabric integration and appear most suited to air-based cooling processes such solar desiccant cooling.

Flat plate collectors have been used with single stage absorption chillers even though collector efficiency is reduced at the temperatures commonly used for driving the chiller. Flat plate collectors are widely available, low-cost collectors used extensively for heating water in the residential and light commercial sectors. 

Evacuated tubes are used extensively in China and are increasingly appearing on the market in Australia. Evacuated tube collectors are non-tracking collectors capable of achieving significantly higher temperatures than that of flat plate collectors. 

Tracking concentrating troughs reflect and focus direct-beam radiation onto a central receiver to achieve the highest temperatures and operate using oil, pressurised water or steam. The higher temperatures from trough collectors allow a more efficient two-stage absorption chiller to be used with a resulting reduction in the required area of the collector field.

With all collector technologies, designs should address the inevitable occurrence of much higher temperatures under stagnation conditions.  

Thermally-driven Cooling

Technologies used for converting solar heat into useful cooling include the following.

Desiccant cooling utilises liquid or solid desiccant material to dehumidify air.  After dehumidification the air is sufficiently dry to enable an evaporative cooling process to cool air well below ambient temperature conditions. This air is then supplied directly to the building. This is an open cycle process where the cooling process utilises water as the refrigerant and air as the delivery media.

While there are relatively few suppliers of these systems, desiccant cooling systems have been used extensively in certain niche applications (e.g. supermarkets) where the ability to independently control air humidity provides additional benefits.

Adsorption chillers perform a closed cycle batch adsorption/ desorption process using a refrigerant and a solid adsorbent to achieve refrigeration.  Refrigeration is used to cool down a secondary refrigerant circuit (chilled water or glycol) to enable the produced cold to be distributed to where it is required.

While there are only a limited number of Adsorption chiller manufacturers, adsorption chiller technology is able to operate with a lower temperature heat source and is more suitable for operation with a dry cooling tower.

Absorption Chillers use a liquid absorbent in a closed cycle process to achieve thermal compression of the refrigerant.  The resulting refrigeration process is used to cool down a secondary refrigerant circuit (chilled water or glycol) to enable the produced cold to be distributed to where it is required.

Absorption cooling technology is mature, low cost and supplied by numerous manufacturers with most commonly available chillers requiring a wet cooling tower.

Absorption chillers are more efficient than other thermal cooling processes which means that less solar heat is required to achieve a given amount of cooling.  Two-stage absorption chillers are even more efficient than single-stage units but require a higher temperature heat source.

Ejector refrigeration uses a thermal compressor (ejector) to compress a refrigerant without the use of any moving parts.  The technology is robust but to-date the technology has not been widely used due to its relatively low efficiency.

Typical heat source matches with cooling technology are summarised below.

Cooling Delivery and System Integration

Many larger commercial buildings use a chiller to cool down a secondary chilled water loop.  Chilled water is then circulated to either:

           (i) Fan coil units, where it is used to cool the air being circulated around the building or

          (ii) Chilled ceilings where chilled water directly cools room air via radiant and convective cooling effects.  

Absorption and adsorption chillers are well suited to these applications and can operate in series with a conventional mechanical chiller, ideally with the absorption chiller providing lead cooling to maximise energy savings.

Where chilled ceilings are being used, the resulting elevated chilled water temperature enables lower temperature solar heat to be used. Similarly, heat rejection with a wet cooling tower is preferable to using a dry cooling circuit when attempting to use low temperature solar collectors.

Some typical integrated system design selections and resulting equipment selection/temperature requirements are illustrated below.

In other buildings, package DX units are often used and chilled water is not included in the base design.  In these buildings solar desiccant cooling configurations are likely to be more attractive.

In most cases, a backup form of cooling is required if comfort conditions in the occupied space are to be adequately controlled.  This can be achieved through installation of;

·        A backup gas burner which provides heat (in place of solar heat) when required.  In this case, no additional chiller/ cooling unit is required which makes this a low-capital cost option.  However, unless the chiller is an efficient two-stage absorption chiller, the greenhouse gas (GHG) emissions from gas firing can reduce the savings that would otherwise be attributed to the solar cooling system

·        A backup (hot or cold) thermal storage tank to defer solar cooling until later in the day when solar heat is otherwise limited.  While this can significantly increase solar fraction, it would be unusual to rely on this as the sole backup source.

·        A backup mechanical vapour compression chiller.  While generally providing better GHG emissions savings, a backup mechanical chiller leads to some duplication and additional capital cost.

Practical Experience

Based on a recent European study, the

(i) solar heat collection and
(ii) cooling process steps

account for around 50% of the cost of a solar cooling installation.  Auxiliary equipment, control and other integration costs account for the remainder.

Typical energy savings from a solar cooling system are around 25% although savings promised at the preliminary design stages have sometimes been eroded by, inter alia, neglected parasitic energy consumption and insufficient attention to part load operation in the control scheme.

Given the maturity of the technology, expert assistance should be obtained to ensure that all the options have been considered and risks have been fully identified.