Heat pumps in public buildings. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Heat pumps in public buildings. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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The main task of compressor equipment in public buildings is cooling, which is necessary either due to climatic conditions or to remove heat from internal sources of lighting, equipment, and personnel. A significant cooling load is required by data centers in service buildings. In this case, as a rule, it is not taken into account that the refrigeration unit used is by its nature a heat pump. Despite the fact that the heat removed from the condenser of the refrigeration unit has a relatively low temperature, its beneficial use provides significant energy savings.

The ratio between the heat removed from the condenser and the power input for both the refrigerator and the heat pump is highly dependent on the difference in evaporating and condensing temperatures. This dependence determines the economic temperature of the water after the condenser of the refrigeration machine in cases where its heat is usefully used. Economically justified is the temperature level of 41-42°C. In this case, the power consumed by the compressor increases slightly compared to the pure refrigeration mode and at the same time it becomes possible not to dump, but to use the heat of condensation usefully.

The most well-known implementation of this concept is that heat from a refrigeration machine that cools the air in the central part of the building is not emitted, but is used to heat rooms around the perimeter of the building, in which heat losses are increased due to the glazing of windows and doors.

Heat from the central part of the building is transferred to the evaporator by means of a water cooling system, and then this energy is transferred to the condenser with the help of a refrigerant and a compressor. Useful heat is transferred to the air cooling tower through a special condenser using a network of heated water, part of the heat is used to heat water or for technological purposes. In winter, when both cooling and heating are required, part of the condenser is used for heating, excess heat is discharged in the cooling tower.

Such an air conditioning-heating scheme is called centralized, using one large refrigerator (heat pump) and room heat exchangers. It can also be used decentralized - with individual heat pumps throughout the building directly in the air conditioning areas. In the latter case, they are connected to an uncooled central water system, in which the temperature is maintained between 15-32°C with the help of an additional water heater and a cooling tower. Each air conditioning unit contains a complete refrigeration and heat pump circuit with a fan for circulating room air, connected to a water system. Water serves as a heat sink in refrigeration mode and as a heat source in heating mode.

Additional heating is required only in case of very cold weather, when most units are in heating mode. Heat is supplied to the water system from a boiler room, an electric outdoor heater, solar energy or a waste heat source. Heat demand is reduced when one or more units have to operate in refrigeration mode. At medium outdoor temperatures, the units on the shady side of the building work for heating, and those on the sunny side for cooling. If approximately 30% of the units are operating in refrigeration mode, then they provide enough heat to the water system, which eliminates the need for the building to receive or give off heat.

In buildings with internal heat dissipation from lighting, computers, etc. and a high level of thermal insulation, year-round local cooling may be required. The heat generated here is transferred to the water system and further to the installations around the periphery of the building, which work for heating during the winter months.

Decentralized systems can also be used in buildings that require cooling during the day and heating at night. If during the day the temperature of the water in the network rises to the maximum temperature allowed for the operation of refrigeration devices, +32°C, then the heat is not discharged to the cooling towers and can be used for heating during part of the heating cycle before turning on additional heating in any form, which is necessary when the water temperature drops below 15°C. The air conditioner starts in the morning when the water is cold and allows efficient cooling, and ends at the end of the day when the water is hot for efficient night heating.

The greatest benefit is obtained when using a heat pump where heating and cooling are required on a large scale at the same time, for example in sports complexes with an artificial ice rink and a swimming pool.

Typically, public indoor swimming pools are major energy consumers, especially in cold climates. The annual energy consumption for public indoor pools is 14000 kWh/m3 of water surface. The required water temperature is around 30°C and the air temperature is slightly higher. The required ventilation rate is from 4 to 20 volumes per hour.

Rotary heat exchangers can be used to use the heat from the exhaust air to heat the incoming air in an energy-saving manner. The use of such heat exchangers is becoming commonplace in swimming pools, however, they recover only part of the heat contained in the exhaust air. Its moisture content is very high, and most conventional heat recovery systems use only sensible heat. Recuperative heat exchangers are able to condense only part of the moisture, and, moreover, relatively small. Latent heat recovery can be greatly improved by using heat pumps, in many cases in conjunction with conventional heat recovery systems.

A typical example of a heat pump installation for a swimming pool complex in Chester (England). The two swimming pools form part of a large indoor sports center and consume the majority of the energy supplied to the building with a design thermal load of 2 MW. Fresh air enters the complex with a flow rate of 46 m3/s, of which 21 m3/s is supplied to the pool hall. The high ventilation rate minimizes condensation in the hall and adjacent rooms and also reduces the smell of chlorine used for sterilization purposes. The total heat load of 2 MW consists of pool water heating, hot water for showers and heating of the adjoining service building. About 3/4 of the total heat consumption goes to ventilation, of which the swimming pool consumes half.

In this case, the most economical is the use of a closed loop with an intermediate coolant in the ventilation ducts together with a heat pump system. Exhaust air, passing by part of the closed circuit, is pre-cooled, giving off a share of latent heat, and then cooled down by 4°C in the heat pump evaporator. The fresh air is first heated by the second half of the closed circuit and then reheated in the heat pump condenser. In the overall heat balance, the closed loop returns about 400 kW and the heat pump a little over 1 MW, leaving a relatively small part of the heat load to be covered by conventional sources.

The use of a heat pump in swimming pools is not limited to air-to-air systems. Sulzer, with extensive experience in the use of heat pumps in swimming pools, combines a range of heat pumps, each with its own purpose. A typical example is the plant in Lindenberg. The indoor pool with a water surface of 315,5 m2 has an air temperature of 30-32°C and a water temperature 2°C lower.

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