Read the following passage and make notes from it using the mapping method. The use of heat pumps has been held back largely by scepticism about advertisers’ claims that heat pumps can provide as many as two units of thermal energy for each unit of electrical energy used, thus apparently contradicting the principle of energy conservation.
Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapour phase in a closed loop. The refrigerant, starting as a low-temperature, low-pressure vapour, enters a compressor driven by an electric motor. The refrigerant leaves the compressor as a hot, dense vapour and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body of air. Now the refrigerant, as a high-pressure, cooled liquid, confronts a flow restriction which causes the pressure to drop. As the pressure falls, the refrigerant expands and partially vapourizes, becoming chilled. It then passes through a second heat exchanger, the evaporator, which transfers heat from the air to the refrigerant, reducing the temperature of this second body of air. Of the two heat exchangers, one is located inside, and the other one outside the house, so each is in contact with a different body of air: room air and outside air, respectively. The flow direction of refrigerant through a heat pump is controlled by valves. When the refrigerant flow is reversed, the heat exchangers switch function. This flow-reversal capability allows heat pumps either to heat or to cool room air. Now, if under certain conditions, a heat pump puts out more thermal energy than it consumes in electrical energy, has the law of energy conservation been challenged? No, not even remotely: the additional input of thermal energy into the circulating refrigerant via the evaporator accounts for the difference in the energy equation. Unfortunately, there is one real problem. The heating capacity of a heat pump decreases as the outdoor temperature falls. The drop in capacity is caused by the lessening amount of refrigerant mass moved through the compressor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant compressed the less the thermal load it can transfer through the heat pump cycle. The volume flow rate of refrigerant vapour through the single-speed rotary compressor used in heat pumps is approximately constant. But cold refrigerant vapour entering a compressor is at lower pressure than warmer vapour. Therefore, the mass of cold refrigerant and thus the thermal energy it carries is less than if the refrigerant vapour were warmer before compression. Here, then, lies a genuine drawback of heat pumps: in extremely cold climates—where the most heat is needed—heat pumps are least able to supply enough heat.