Page 108 - Low Temperature Energy Systems with Applications of Renewable Energy

Effective use of heat pumps for various heating applications 97

assumed that the relative humidity of air in the room should be within the range of

60e80%, the room temperature is 27 C, and the water temperature is 25 C. In the
absence of patrons, at a relative humidity of 70%, about 64 g/h of water will be evap-
2
orated from each square meter at the temperatures mentioned above, i.e., 64 g/(m h),
2
whereas at a relative humidity of 60%, 91 g/(m h) will evaporate. A higher humidity
in the room will result in vapor condensation on the building walls.
When there are bathers in the pool, the evaporation will be increased by increasing
the evaporation surface because of people’s bodies, wet walls and floors, water agita-
tion, people’s breathing, etc. At 70% relative humidity and a small number of people
2
2
(15e20) in the pool, evaporation increases to 162 g/(m h), and up to 475 g/(m h)
2
assuming each person in the filled pool occupies 1.5 m . Thus, the average evaporation
2
rate ranges from 100 to 300 g/(m h).
The amount of heat (thermal power) consumed for evaporation will be found as the
product of the moisture amount for the whole area of the pool times the heat of evap-

oration, namely, 2444 kJ/kg at 25 C. Heat losses of the pool area into the environ-
ment are determined by traditional methods. It should be noted that air warms the
pool water, because it has a higher temperature. The heat transfer coefficient may
2
be taken as 6 W/(m K). When the water in the pool is colder, which occurs mainly
at night when the system power is lowered, the heat transfer coefficient may be taken
2
as 20 W/(m K). Therefore, it is important to cover the pool when not in use. When
changing water in the pool (e.g., according to the norms of France, 5% of water should
be changed every day, and all of it should be changed out once every three months),
the power for water heating is determined by the water heat capacity and the difference
in water temperature in the pool and at the entrance to the heating plant. Heat losses to
the soil at constant pool filling are small and can be found according to empirical
studies using a theoretical coefficient of heat transfer in the “wall-ground” system
2
of about 5 W/(m K).
To reduce the humidity of air in the pool area, conventional ventilation of the room
3
is often used. The volume flow rate of fresh air (m /s) is estimated by the following
relation:

_
V ¼ _ m ½ðd room d input Þ,r Š (3.2)
air
where _ m ¼ evaporated water rate, kg/s; d room ¼ absolute air humidity in the pool area;
d input ¼ absolute humidity of atmospheric air entering the room, kg ha /kg da
3
[ha ¼ humid air, da ¼ dry air]; r air ¼ density of fresh air from outside, kg/m .
2
3
The air flow rate reaches 20,000 m /h for a pool with an area of 250 m . This air
requires heating which consumes a significant amount of thermal energy. To reduce
its consumption, heat recuperators are installed using thermosiphons or a heat pump
(HP) may be used when the pool is occupied. Fresh air is warmed by heat from a
HP condenser. In order to maintain comfortable conditions, it is recommended
installing one more HP for heating fresh air both in combination with thermosiphons
and without them. The HP, which dries the air, sucks it out of the pool area and ejects it
into the room. The air passing through the evaporator cools down and the moisture