Page 74 - Low Temperature Energy Systems with Applications of Renewable Energy

Characteristics of low-temperature energy sources for heat pumps 63

promising methods of energy saving. In addition it is important to note that the effi-
ciency of this system depends greatly on the heat transfer intensity when using the
heat of low-potential drain heat. In compact heat exchangers with metal head collec-
tors, the heat transfer intensity is much greater than in coiled-tube soil heat exchangers
made of polymer pipes with which headless concrete sewers are equipped.

2.4.3.2 Conventionally pure building sewer drains

Building sewer drains, the temperature of which can reach 32 C, can be used as lower
heat sources for local heat pumps, if there are hot water supply systems. For this pur-
pose special heat exchangers, operating together with a heat pump in a residential
building hot-water supply system, may be located in the basement. The heat exchanger
design should not obstruct or prevent the natural movement of waste liquid in the
sewer pipes. A possible solution may be a “pipe-in-pipe” heat exchanger placed
around the casing of the sewer pipe and fed into the formed space among heat-
carrier pipes from the heat pump evaporator. Heat extraction efficiency in such a
heat exchanger is not high, but may be intensified if use can be made of a separate
drainage system of warm, so-called “conventionally pure” waste water from baths,
sinks and kitchens, and separately, cold and more polluted water from the toilets.
The hot water supply system is characterized by the fact that most of the thermal
energy of the hot water is almost never used. The thermal energy in the sewer drains
is not much less than the heat amount that is spent for water preparation in the hot wa-
ter supply systems. Thus, the thermal energy of conventional pure waters can be used
as the heat supply for heat pumps.
At the same time one must note that the wasted thermal energy is not sufficient to
provide heating to a building. That is why if the heat pump using ambient air heat is
used to heat a building, conventional-pure waste waters can be collected in a special
accumulator (either directly or after using the heat pump in the hot water supply sys-
tem) and crystallization heat of this water can be used for preliminary heating of the
ambient air in front of the heat pump in a cold season. The efficiency of this variant
of building heat supply greatly increases when lower rates for night electricity is
available.

2.4.4 Waste water heat of industrial enterprises

Waste heat from water at industrial enterprises can be of various origins but the most
widely used are cooling waters of technological and electrical power generating sta-
tions. In closed systems of service water supply, this water goes to cooling towers
where the waste heat goes into the surrounding air. The properties of waste water
are characterized by relative stability so that the heat of such water may be quite effec-
tively used in heat-supply heat pump systems (Fig. 2.13).
Water heated in a piece of technological equipment 1 is fed by the pump 3 into the
intermediate tank 4 through which water is pumped by pumps 5 and 6 into the cooling
tower 2 and evaporator 7 of the heat pump. When it is required to provide heat to the
user 11, the heat pump is used and the heat from the heat pump condenser 9 starts