Cooling in buildings is usually achieved by circulating chilled water throughout the building by means of pipes and using fan coil units that blow air over these pipes in the individual rooms. The chilled water is provided by chillers which require an energy input.
Chillers are essentially heat pumps operating on a refrigerating cycle. In nature, heat will flow from a region of higher temperature to regions of lower temperature. This phenomenon can be exploited to obtain a work output. In fact, this is known as the heat engine. Conversely, in a heat pump, heat is transferred from a region of lower temperature to a region of higher temperature and thus a work input is required. Heat pumps perform well inherently but one way of improving their performance further is by lowering the temperature of the region to which they reject heat as this will require less work to transfer the heat energy.
The conventional heat sink for chillers is ambient air. However, in the summer season when cooling is needed the most, the temperature of air is usually high and therefore it limits the performance of the chiller. This results in more energy consumption for a given refrigerating effect. With rises in energy costs and increased concern over the by-products of combustion, the use of saline water as the heat sink in chillers could be a more attractive solution.
Saline water in general exhibits lower temperatures than air in the summer season when cooling is necessary. It could therefore improve the performance of the chiller, reduce its energy needs thus making it more efficient and ultimately reduce operating costs. Additionally, saline water as a heat sink is renewable. Several cooling systems found in facilities on the coast of St Julian’s are already making use of chillers cooled with saline water as opposed to air.
The main components in such systems are the seawell from which saline water is obtained, the pumps, the heat exchanger, the chiller and the outflow through which higher temperature saline water is rejected to the sea.
An additional heat exchanger may be included to harvest the heat from the saline water after it has passed through the chiller and transfer it to domestic water to be used as hot water in showers and kitchens. It is important to keep in mind that chillers cooled with saline water are constrained by location – the site needs to have access to saline groundwater at reasonable depths. Bigger depths will increase the cost of drilling the seawell and pumping costs, thus making the system unfeasible. The high capital costs of such systems mean that they are more feasible for use in larger buildings with larger cooling requirements.
Heat pump performance is characterised by a parameter known as the Coefficient of Performance (COP). A typical air cooled chiller has a COP of 4.0. This means that for every unit of energy used, four units of cooling are produced. For a water cooled chiller system COPs equal to 5.9 are not uncommon. This improvement in performance is rather drastic and could translate into an energy saving potential of 63.3 GWh of electricity per year and a total of 55,733 tonnes of carbon dioxide emissions avoided per year. 63.3 GWh represents approximately 2.85% of the generated electricity in Malta per year.
The research work disclosed in this publication is partially funded by the Strategic Educational Pathways Scholarship (Malta). This Scholarship is part-financed by the European Union – European Social Fund (ESF) under Operational Programme II – Cohesion Policy 2007-2013, Empowering People for More Jobs and a Better Quality Of Life.