Page 58 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low- 2

temperature energy sources for

heat pumps
2.1 Ambient air usage in space conditioning

Although ambient air is the most available of all possible low-potential heat sources, its
temperature potential is not high enough to permit its use as a heat source for heat pumps
in cold seasons. The coefficient of performance (COP) is lower than an alternative effec-

tive system when the temperature of ambient air is lower than 3to 5 C [1e3].The
corresponding comparisons of water-heating heat pumps having conventional boiler
houses and condensing boilers (see Chapter 1) are shown in Ref. [4].It is also
shown that, when comparing heat pumps with a conventional boiler house, significantly

low limiting temperatures ( 10 to 15 C) are possible only for low-temperature
radiant floor heating systems. Critical ambient air temperatures for heat pumps take

the values of 0e10 C for low-temperature and 2e7 C for conventional radiator heat-
ing systems.
An important conclusion from Ref. [5] is that only subtropical climates are consid-
ered suitable for effective application of heat pumps using ambient air in heating
systems without requiring supplementary energy sources. Various kinds of reversed,
split-type heat pumps operating for cooling in summer and for heating in cold seasons
are the simplest technical solution in this case.
Moreover, in summer the temperature potential of ambient air is relatively high and
it should be used for hot water systems in many cases. For instance, often a boiler
house does not operate in summer, leading to problems of hot water supply. These
problems can be solved by means of an “air-water” heat pump installed in the facility
where hot water is normally prepared (Fig. 2.1). Besides the conventional equipment
of the heating plant (items 1e7), the system includes a heat pump 8, the condenser of
which heats the water that is then circulated by the pump 10, located between the
condenser and accumulating tanks 9.
The heat potential of ambient air can be used more effectively in the heat pump if
other heat sources besides air ambient are also used. For example, an ambient air
receiver can be made in a heat exchanger (HX) placed on a house roof. The heat
from the HX is transferred by means of glycol circulating between the HX and the
heat pump evaporator. Heat transfer intensity increases from air to glycol under the ef-
fect of wind; furthermore, the HX receives solar radiation energy that makes heat pump
operation more effective.
Ambient air heat for heating systems can be used under appropriate conditions by
combining it with the heat of ventilating exhaust of the same building. As an
example, a positive experience of using such a system was obtained when

Low-Temperature Energy Systems with Applications of Renewable Energy
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