Page 60 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 49

2.2 Building and construction ventilation air

2.2.1 Residences and multi-story apartment buildings
2.2.1.1 “Air-air” ventilation systems
In places with climates that do not conform to ambient temperatures in the range of

5e7 C for best performance during heating seasons, it is not feasible to install off-
the-shelf ambient-air heat pumps. Nevertheless, even in winter, sources of
low-potential heat such as ventilation discharges from industrial and residential establish-
ments fall into the acceptable range.
Private homes (or small houses) and collective multi-story buildings where there are
people (e.g., apartment houses, schools, universities, hospitals, medical centers, day-
care centers, offices, etc.) all require ventilation at rates from 80% to 100% of room
volume per hour. In fact, the rate of ventilation might be as high as 300e400% in busi-
ness centers.
Ventilation air in modern buildings is ejected or absorbed by ventilators. Such a
system of air ventilation permits not only the use of heat pumps, but also heat ex-
changers so that the fresh air stream can be heated directly by the exhaust stream. If

the temperature of exhaust air is up to 20 C, the heat pump at such temperature
will have a COP between 3 and 4. In older houses this system cannot be effectively
used unless they have separate inlet and discharge ventilation channels. Heat pumps
are used mostly for hot water supply and for heating fresh cold air.
The most economical use of heat pumps is after-air cooling in a heat-exchanger,
operating on the principle of thermosiphons, conventional heat-exchangers, or
Ljungstr€ om regenerative type heaters with metallic heat-accumulating chambers, be-
ing rotated in contact with warm and cold air in sequence. Exhaust air is first cooled
in the heat exchanger and then in the heat pump evaporator. Fresh air is first heated
in the heat exchanger and then directed into the heat pump condenser. It is generally
assumed that for individual buildings, the specific energy requirement for ventilation
3
heat transfer is less than 0.3 W/(m K) which corresponds to a circulation rate of
70e90%. The heat pump should have a defrost system that starts operating when

the ambient temperature is approximately 3e5 C.
The heat pump assembly in private dwellings can be sited under the roof taking into
account that the size of the heat pump is small. Noise protection should be provided.
For this purpose the heat pump should be placed on additional flooring consisting of
double-layered wood with noise protecting isolation between them. The additional
floor dimensions must be 2e4 times larger than the heat pump footprint area. The
heat pump equipment for the house can be installed in the garage or cellar as well,
thereby providing vibration and noise protection. When cooling the air, some conden-
sate is formed owing to the humidity of the air, and it is necessary to collect and
remove it from the building or into a special holding tank.
Heat pump heating of multi-story apartment buildings using air requires an addi-
tional conventional heat source which must be decentralized. The heat pump can
cool and dehumidify the air in summer.