Buổi 92: Reading – Diagram labelling + T/F/NG: máy bơm nhiệt hoạt động thế nào?

Moving heat rather than making it

(Bài đọc tự viết; mô tả đơn giản hóa, hộ gia đình là giả định.)

A Most traditional heating systems work by burning fuel. A gas boiler, for instance, burns gas to heat water, which is then pumped around radiators. A heat pump works on a fundamentally different principle. Rather than creating heat, it moves heat that already exists from one place to another – usually from the air or ground outside a building into the rooms inside. This may sound unlikely on a cold winter day, but even air well below freezing contains a considerable amount of heat energy.

B The process depends on a special fluid called a refrigerant, which circulates continuously through a closed loop of pipes. The key property of a refrigerant is that it boils at a very low temperature. In the outdoor unit, air is blown across a coil of pipes known as the evaporator. Although the outside air may feel cold, it is still warmer than the refrigerant flowing through the coil, so heat passes into the refrigerant, which evaporates and becomes a low-pressure gas.

C This gas then travels to the compressor, the part of the system that uses electricity. By squeezing the gas into a smaller space, the compressor raises both its pressure and its temperature considerably. The hot, high-pressure gas then flows to a second coil, the condenser, located inside the building or connected to the heating system. Here it releases its heat into water or air that is circulated around the home. As it loses heat, the refrigerant condenses back into a liquid.

D Finally, the liquid passes through an expansion valve, which lowers its pressure sharply. This causes its temperature to fall well below that of the outside air, ready to absorb heat again in the evaporator. The cycle then repeats.

E The great advantage of this process is efficiency. Because the electricity is used to move heat rather than to generate it directly, a heat pump can deliver several units of heat for every unit of electricity it consumes. By comparison, an electric heater that converts electricity directly into heat can, at best, deliver only one unit of heat per unit of electricity. Heat pumps are therefore considerably more efficient than direct electric heating, and, depending on the source of the electricity, they may produce far lower emissions than gas boilers.

F There are, however, limitations. Heat pumps become slightly less efficient as the outside temperature falls, since there is less heat available to extract. They also tend to deliver heat at a lower temperature than gas boilers, so they work best in buildings that are well insulated, where a steady, gentle heat is enough to keep rooms comfortable. In poorly insulated homes, larger radiators or underfloor heating may be needed.

G Cost is another consideration. The upfront price of installing a heat pump is generally higher than that of a new boiler, although running costs may be lower, depending on local energy prices. Consider the hypothetical Tran family, who replaced an old boiler in a recently insulated house. Their installation cost more than a replacement boiler would have, but they found the house noticeably warmer and more evenly heated than before. Whether they will save money overall, they admit, will depend on how electricity prices change in the coming years.