Environmental Engineering Reference
In-Depth Information
Although combustion of natural gas might seem to be the most efficient way to gen-
erate heat, there are other methods that use the heat contained in the environment in
a very clever way such that more heat can be obtained than that of either combustion
or electrical resistance. Consider the idea of a heat pump as described next.
5.3.2
Heat Pump
A heat pump is an energy system that uses electricity to move heat from a low tem-
perature source to a high temperature source. Since the heat pump uses electrical
energy to move heat from the environment to the desired location, it can obtain ther-
mal efficiencies defined by Eq. (5.1) much greater than 1. Because it is not usual to
have an efficiency greater than 1 (or 100 %), the efficiency of a heat pump is defined
in terms of a coefficient of performance:
Energy obtained
Coefficient of Performance (COP) = Electrical energy supplied
(5.2)
For a device that uses electrical resistance as the heat source, Eq. (5.2) would give
a COP value of one. For a heat pump however, COP values of 3, 4 or even 5 are not
uncommon. A COP value of 4 means that 1 kW of electrical energy supplied will
generate 4 kW of heat. Remarkably, this would be four times that of a device that
uses electrical resistance heating and much more than would be possible by burning
fossil fuels!
Before explaining the components of a heat pump, it is helpful to compare the
heating that can be supplied by combustion, electrical resistance and a heat pump.
Consider the conversion of 100 units of fuel to heat as shown in Fig. 5.3 . In combus-
tion, some amount of the heat is lost to the environment, so that from 100 units of
fuel, 80 units of heat can be obtained. In electrical resistance heating, the thermal
efficiency given by Eq. (5.1) is close to 1 and this would give a COP value of 1 if
Eq. (5.2) were used. However, electricity must be generated and power plants are gen-
erally only about 40 % efficient. Thus, for electrical resistance heating, 100 units of
the original fossil fuel provide 40 units of electricity which gives only 40 units of heat.
As also shown in Fig. 5.3 , a heat pump uses the electricity in a device to move
heat from one place to another rather than supplying heat directly. From 100 units of
fuel, 40 units of electricity are produced from a power plant as in the case for elec-
trical resistance heating; however, a heat pump with a COP of 4 moves 120 units
of heat from the environment to give a total of 160 units of total heat. Thus, a heat
pump is a device that allows highly efficient use of energy resources.
5.3.3
Working Fluid
To move heat from the environment to a target, a heat pump uses a working fluid
that flows through various devices in a cycle (Fig. 5.4 ). Electricity is used to supply
the work ( W ) required to move the fluid through the various devices in the cycle.
Search WWH ::




Custom Search