Solar Cooling Air-Conditioning: Using the Sun as the Driving Source

By Inga Doemland, ANU CECS Solar Thermal Group, inga.doemland@anu.edu.au. Supervisor: Mike Dennis

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Fig. 2: Electricity demand and cost on a hot summer day in QLD

Peaks during trading hours and after work

Price peak

indicates high

demand

Fig. 1: Shopping centre Newcastle, 300 kWr Absorption chiller, concentrating troughs and hot water tank.

Fig. 3: Solar insolation on a hot summer day in Brisane

Shift between peak insolation and peak air conditioning demand

Introduction Solar Cooling sounds confusing at first. Heat from the sun is used to drive an air conditioning process in order to create an acceptable indoor climate just like standard vapour compression air conditioners do.

But standard air conditioners use electricity from the grid to generate the required amount of cold. About 5.7 million units are installed in the commercial and residential sector in Australia with a capacity of approximately 53 GW [3].

In comparison to that there are worldwide approximately 500 solar cooling plants installed which are mainly single pilot plants in the commercial sector in Europe. No standard and optimal configuration is available and the cost is high.

Optimizing Solar Cooling systems Clear disadvantages are the small amount of installations, the lack of experience and the uniqueness of every system and therefore the high cost. Furthermore collector and tank size as well as cooling capacity influence the system cost and are to be optimized.

The insolation in Australia provides the necessary heat, but only well designed systems are able to be cost competitive. As can be seen in Fig. 3, there is a shift between maximum insolation and cooling demand. Usually a hot water tank is used to overcome this mismatch. The technologies

Generating low temperature heat using a high temperature heat source is not a new invention and large thermal chillers have been commercially available for a long time. Also small scale systems for the domestic sector entered the market during the last few years.

Generic systems which seemed to have established themselves consist of a solar collector, dependent on the required temperature, a hot water storage tank and either of the following

  Ab- or Adsorption chiller & heat rejection device (cooling tower) (Chemical process to produce cold water)

  Solid/ liquid desiccant systems (Integrated in the supply/ exhaust air stream)

Also possible is a combination of a standard electricity driven vapour compressor combined with photovoltaic panels.

Why Solar Cooling? Air conditioning accounts for a high fraction of the load on the

electricity grid during trading hours. It’s mainly caused by the commercial sector including office buildings, shopping centres, administration buildings etc. The second big impact is generated after working hours by the domestic sector in order to cool down living and sleeping areas. These peaks are shown in Fig. 2.

Outlook An increase in small and large scale solar cooling applications can certainly alleviate the electricity grid . Therefore the performances of the systems have to be rated to establish consistent standards. Building structures play an important role on the system design and need further investigation.

References [1] Australian Energy Market Operator. (2011, 21 June 2011) Aggregated Price and Demand: 2006 - 2011. Available: http://www.aemo.com.au/data/aggPD_2006to2010.html#2010

[2] TRNSYS simulation, version 16_1, weather data files TMY2, Meteonorm distribution.

[3]Department of Sustainability, Environment, Water, Population and Communities,. (2011, 21 June 2011). The Refrigeration and Air conditioning industry in Australia. Available: http://www.environment.gov.au/atmosphere/ozone/publications/cold-hard-facts.html

Next to this, it is possible to utilise the thermal mass of buildings, to shift peak cooling demand to hours more suited for solar air conditioning.

Thermal mass can be used to shift air conditioning demand in the evening/night to allow the cool night breeze to cool the walls down or to use the cheaper night time electricity tariff to run air conditioners at lower cost. Usually a climate zone with an high diurnal temperature range is required.

On the other hand internal thermal mass can be used to pre-cool the building in the morning to reduce the peak in the afternoon. This could reduce collector and tank size and therefore the cost.

Fig. 4: Indoor operative temperature for two different building structures

Shift between operative temperature peaks

Fig. 4 compares the operative indoor temperature of two different construction types: insulated weatherboard vs. Insulated double brick .The shift indicates how thermal mass acts. The correlation between temperature and construction type can be modelled using the transient simulation software TRNSYS.