Page 135 - Low Temperature Energy Systems with Applications of Renewable Energy

124 Low-Temperature Energy Systems with Applications of Renewable Energy

On the basis of the research carried out in Ref. [24], the following conclusions were
drawn:
1) HPS with a GT compressor drive and waste heat recovery for use in the furnaces of water
boilers provides the HPU a greater share of load b of the HPS than one with an electric or
compressor drive from diesel engine.
2) The lowest specific consumption of conventional fuel is provided by the HPS with a GT
driving the compressor. The HPS with an electric drive and the one with a diesel approach
it in terms of efficiency. The water boiler with a GT is characterized by the largest specific
consumption of conventional fuel.
3) The highest saving of conventional fuel is observed for the HPS with a GT driving the
compressor (up to 45.3%). The savings of conventional fuel for the HPS driven by a diesel
is up to 36.7%. The savings of conventional fuel at the HPS with an electric drive does not
exceed 26.2%, but is higher than that of the conventional fuel of the WB with a GT.
4) Thus, the HPS scheme with the compressor driven by a GT has the highest overall
effectiveness.

3.5.4 Impact of condensers and evaporators on heat pump
efficiency

The arrangement of condensers and evaporators in heat pump units can have a signif-
icant effect on the thermal performance the HPU. The main alternatives are series and
parallel connections. Although a series connection of condensers of several HPU in-
volves higher energy consumption for pumping the working fluid, this higher power
consumption is offset by significant savings on power to drive the compressors. Using
a series connection of HPU condensers with a parallel evaporator connection ensures
working fluid heating to a higher temperature with lower energy consumption (a higher
COP) or allows the working fluid to be heated to a higher temperature with the same
energy consumption as with a parallel condenser connection. The analysis of various
arrangements of HPU connections was performed in Ref. [24].
Figure 3.39 shows the circuit of a HPU with series connections for the condensers
and boilers, and parallel connections for the evaporators.
The temperatures of direct and return circuit water in the system of heat supply are
determined by the temperature schedule that depends on the temperature of ambient
air. The value of the HPU load fraction in the HPS (b) depends on the temperatures
of the direct and return circuit water, which, in turn, depend on the temperature of
ambient air. The optimal values of b for HPS circuits with different types of drives
and arrangements of evaporators and condensers at different modes of heat network
operation were determined in Ref. [25]. A method was proposed for determining
the proportion of thermal power load on the HPU, b, that depends on the temperature
of ambient air, by taking into account the complex influence of HPU condenser
connection circuits, the type of the HPU compressor drive, and the modes of HPS
operation.