6.1 INTRODUCTION
In thermodynamics, several major devices related to energy conversion are
introduced, including the internal combustion engines (ICE), steam turbine,
boiler, heat exchanger, and heat pump. Thermal efficiency is a dimensionless
performance measure of these devices. In general, thermal efficiency is the
fraction of energy addition in the form of heat or thermal energy converted to
useful output, as shown in Fig. (6.1), given as a percentage value. The
nominally Otto-cycle ICE in automobiles can reach about 30% efficiency at
the flywheel. Rankine-cycle steam turbine thermal efficiency can be as high as
41%. In heat pump cycles where heat rejection in the high-temperature side is
the useful output, the efficiency is usually defined as the ratio of the rejected
heat to the compressor work input, commonly called the coefficient of
performance (COP). Refrigeration moves heat from a confined space and
dissipates it in the atmosphere. Heat pumps move heat from one spot (often
from the atmosphere or underground) to a home of business. Residential
refrigerators and air conditioners are based on a vapor-compression
mechanical refrigeration cycle. The former generally has a COP over 1 in
practice, while the latter may have a COP over 3 to 5. The COP of heat pumps
is generally higher than that of their refrigerator counterparts.
