Absorption Cooling & CHP
Some sites that might consider installing CHP plant also have a large and continuous cooling demand, for example for process cooling or for air-conditioning extensive computer suites. In these instances, it is worth considering absorption cooling.
Furthermore, if a site is to maximise the financial benefits of CHP, all the electrical and heat output of the plant must either be used on-site or be sold elsewhere.The size of CHP plant installed will, therefore, be carefully determined in relation to the level of electrical demand and the heat to power ratio.
Because sites usually have a greater demand for electricity than for heat, it is the heat demand that, in most cases, determines the CHP unit’s size. As a result, most CHP units produce less electricity than the electrical base load demand of the site they serve. Where these sites also have a cooling requirement, absorption cooling offers two potential advantages:
- Most of the electrical load required to meet the cooling demand is converted into a heat load, thereby reducing the electrical demand and increasing the options for heat use.This can materially alter the site’s heat to power ratio, perhaps turning a hitherto marginal case for CHP into a viable option. In some cases, it may even encourage specification of a larger CHP unit that will economically generate more electricity.
- The additional heat load allows the plant to operate more efficiently by extending profitable CHP running time and, to some extent,‘ironing out’ the seasonal peaks and troughs. This can apply to both new and existing CHP units.
Most standard chiller designs require heat in the form of steam or hot water, and there is a strong link between the type of absorption chiller selected and the CHP prime mover envisaged or already in place:
Steam at 7-9 bar gauge – typically the product of a gas turbine with heat recovery boiler – is best used in a double-effect absorption chiller. This type of chiller can also accommodate hot water at 170-180°C, although these units are not widely available.
- Hot water or low-pressure steam is usually more appropriate to a single-effect absorption unit. The necessary level of heat is readily supplied by most models of engine CHP (hot water at 85-90°C), with larger engines producing water at temperatures of up to 130°C.
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