Calculated cooling

The search for environmentally friendly building services throws up a steady stream of options. Their reliability and practicality is not always cost-effective, however. System sizing is one of the most crucial factors. If over-capacity is eliminated, capital, running and maintenance costs are all lowered, and energy use is reduced.

Desiccant cooling falls into this category. A relatively new, refrigerant-free technology which can provide both cooling and dehumidification, it is an open heat-driven cycle which uses a desiccant wheel and thermal wheel in tandem. A wide range of sources can be used to provide the heat required for reactivation, including gas, hot water, waste heat and solar thermal energy.

A recent research project undertaken by Gaia Research and the University of Leeds’ school of civil engineering set out to produce a software program that helps designers size a gas/solar desiccant system. The result is the Solar Desiccant Calculator.

The calculator program performs two functions. First, it can be used to size desiccant cooling equipment based on summer and winter conditions. Second, energy analysis can be completed for sample winter and summer days.

The program calculates potential energy savings, the reduction in greenhouse gas emissions, and running costs.

Meteorological data for three UK cities – London, Lincoln and Edinburgh – has been incorporated, representing the south-east of England, the east midlands and the midlands of Scotland respectively. The program’s calculations are also based on the monitored performance of installed plant in these regions.

The calculation process

The program assumes the incorporation of solar pre-heating coils before the regeneration coil on the return air side, and before the heating coil on the supply air side. However, it is possible to run the calculator without the inclusion of the solar coils.

The user must navigate through a series of screens that require basic system information. The initial screen provides the option of either starting a new design calculation or reviewing previous design data. The latter is designed to enable further investigation of energy performance under different environmental conditions.

For a new calculation, individual data screens are provided in turn for mechanical equipment data, solar regeneration coil data and solar pre-heating coil data. On each of these screens, five dialogue boxes enable the user to key in information, which ranges from supply air volume flow rate to the U-values of the regeneration and pre-heating coils.

Two options are given to the user: equipment sizing or energy calculation. The first enables sizing of the system under typical hot and cold day conditions. This step can be bypassed and the second option chosen if the user is confident about the design and simply wishes to examine its energy consumption over a typical daily cycle.

To size a system, environmental data must be entered, such as external and internal air conditions for the summer and winter cases. This produces a set of intermediate results, with temperatures listed in box and diagrammatic formats. Energy costs of the system can then be calculated.

A final screen lists the simulation details and results. This covers the amount of gas and electricity delivered, the cost per metre floor area, the amount of primary energy consumed, and carbon emissions.

The speed at which this whole process can be carried out – in under 30 minutes – is an attractive element of what promises to be a very useful program.