Information on some important Energy-Saving Equipments

(i) Hot cupboards: with a double skin of metal, the space between is filled with an insulating material.

(ii) Microwave ovens: require a low degree of energy compared to tradi­tional cooking methods.

(iii) High-pressure steamers: can be used in place of billing pans and steam­ing ovens.

(iv) Induction cookers: on average use 46 percent less energy than a tradi­tional electric hob; a La carte kitchens can save up to 64 percent. Further savings can be made because ventilation and extraction are only needed a little or not at all.

(v) Steamers: steaming foods is an energy-efficient process and has the added advantage of retaining a larger proportion of nutrients in food than boiling.

(vi) Combination ovens: i.e. forced-air/convection, steaming/convection, mi­crowave/convection can result in savings of energy when used correctly.

The above measures are designed to minimize energy wastage as far as pos­sible. However good these programmes of energy conservation through improved efficiency, some energy will always be wasted, usually in the form of heat.

This heat will be wasted in association with the flue gases resulting from the combus­tion process, in hot air extracted from kitchens, laundries, sport and leisure centres and in waste hot water. It may be possible to recover a proportion of this waste heat.

The recovered heat can then be recycled to provide part of the heating requirement for domestic water supplies or for heating systems. With all these systems, great care is required at the design stage if the potential benefits are to be obtained. The feasibility of recovering heat depends upon satisfying the follow­ing criteria:-

1. The waste heat must be sufficient in quality (purity and temperature) and quantity.

2. There must be a convenient use for that waste heat (not too far away and closely linked in time to the source)

3. The cost of recovery must be greater than the combined capital and running costs of its recovery.

The mechanics of recovery are reasonably straightforward. A heat exchanger is normally required to separate the waste heat from its course (combustion prod­ucts, exhaust gases or waste), to transport the heat to where it can be reused and to transfer the recovered heat to the chosen application (hot water supply, swimming pool water, warm air heating system, etc.).

There are number of types of heat exchangers. Thermal wheels, which consist of a wheel made out of heat-absorbing mesh which rotates between inlet air and outlet air, can be used where the outlet air is reasonably clean and free from odours and grease and can recover up to 70 percent of waste heat. They have been used in swimming pools to recover hot moist air which collects near to the roof of the pool and to recycle the heat into the make-up inlet air.

Where the air is contaminated with odours and or smoke, a system which separates the contaminated exhaust air from the clean air is essential. One way of doing this is to use an air-to-air heat exchanger where the two air flows are separated by metal plates which allow conduction of heat across the plate, while protecting the supply a^;r from contamination in the waste air.

An alternative to the air-to-air heat exchanger, particularly where the source of waste and location of reuse are some distance apart, is to use a run-around coil. In this type of heat exchanger, water (or some other heat transfer fluid) is pumped through a pipe going from the source of waste heat to the point where the heat can be-used before returning to the starting point.

Where wasted heat is at a low temperature, such as the extract from canopies room ventilators and swimming pools, this limits the usefulness of the recovered heat.

Although large volumes of heat may be extracted, it is often referred to as ‘low-quality heat’ because it is diluted with large volumes of air. To recover heat from a large volume of air which is at a low temperature is difficult and often not worth the cost involved.

One of the problems with passive systems, such as run- around coils, is that the maximum temperature which can be delivered is that of the extracted air. Thus, if air from a canopy is at a temperature of 300° C, the maximum theoretical temperature which can be transferred to the water in a pre­heat tank is also 300° C.

If higher temperatures are desirable, then an active device, such as a heat pump is required. This works rather like a refrigerator, extracting heat from a source at low temperature and delivering the heat to a region at a higher tempera­ture.

In this way, a heat pump can raise the temperature of waste heat, but in order to do this an additional form of energy, usually in the form of electrical or gas- powered compressor is required.

For example, air can be extracted from a canopy at a temperature of 300° C. and be used to preheat domestic hot water supplies to a temperature of 300° C. whether this is effective or not depends on two factors.

(i) The first of which is the ratio of energy recovered to energy which must be supplied to the heat pump. This ratio is known as the ‘Coefficient of Performance’, which is typically of the order of 2.5 to 3. A figure of 3 would mean that for every kilowatt-hour of electricity or gas supplied to the heat pump unit a output of 3 kwh of heat would be supplied to the heating or domestic hot water system.

(ii) The second factor is the relationship between the quantity of heat wasted, together with the time at which this waste heat is generated and the lime at which the recovered heat can be re-utilized. Heat is usually expensive to store for long periods of time.

As with any investment, the financial feasibility of an investment in heat- recovery measures will depend on the payback time for the capital installation based or fuel savings in supplying the space heating, pool heating or domestic hot water. As energy becomes increasingly expensive, this paybacks time will, short and investment is likely to return a greater benefit in years to come.