Page 217 - Low Temperature Energy Systems with Applications of Renewable Energy

204 Low-Temperature Energy Systems with Applications of Renewable Energy

5.6.1 Single-stage heat pumps

Using geothermal waters in the temperature range of 30e60 C as low-energy heat
sources for heat pump units (HPU) allows raising the HPU condensing temperature

to 90e95 C, leading to a high coefficient of performance, COP. According to the
thermophysical properties of heat pump working fluids, as the condensing temperature
in the cycle increases, the latent heat of condensation decreases and the inner irrevers-
ible losses due to throttling increase, which causes a reduction in COP. It is possible to
provide a high temperature for heat transfer (85e90 C) using a single-stage heat

pump system by choosing high-temperature working fluids as refrigerants. At the
same time, the total heat output may decrease, which would result in smaller heat ex-
changers and compressors for the HPU. In any case, the selection of the working fluid
for the HPU, subject to predetermined temperature limits of a thermodynamic cycle,
will always involve a compromise. This poses a challenge for the system designer
to achieve both high performance and high temperatures for heating applications.
In this connection, by using a numerical simulation of the thermodynamic charac-
teristics of a geothermal heat pump with a given technological scheme we can find a
working fluid which will provide effective performance under high-temperature con-
ditions. The influence of the particular technological scheme of the HPU elements on
the heat transfer efficiency will be considered.
The efficiency of using a low-potential geothermal fluid in a HPU also depends on
its final temperature. This is achieved by the use of one HPU or two or more HPUs
connected in series. With the successive movement of thermal water through the evap-
orators, the HPU allows the process of evaporation of the working fluid at different
temperature levels, which leads to an increase in the total COP and saving energy
on the compressor drive.
Depending on the parameters of the geothermal fluid (flow rate and temperature)
and the customer’s requirements for the final temperature, up to three HPUs can be
included in the system. Cascade or multi-stage HPUs can decrease the thermodynamic
losses while raising the complexity and initial cost of the system. We will examine this
generalized case, and later specialize it to the simplest single-stage system.
It is possible to expand the working range of geothermal heating systems with
HPUs using several units in a series counterflow arrangement; see Fig. 5.28A. Water
heating in a condenser and cooling of geothermal water in an evaporator are realized in
stages, in which each successive cycle operates at a higher temperature level for evap-
oration and condensation of the refrigerant. Thus, within specified temperature limits
of the basic working cycle, as the number of stages increases, the performance of the
HPU approaches the ideal limit of the Carnot cycle. In the unattainable limit of an
infinite number of stages, an effective cycle having non-isothermal processes of evap-
oration and condensation would be reached. The closeness of the approximation to the
Carnot cycle is measured by the ratio of the actual HPU coefficient of performance
COP HPU to that of the theoretical Carnot HPU cycle COP Carnot [37e41].
Fig. 5.28B shows a single HPU having a multi-stage compressor which can be
configured with interstage cooling to reduce the overall work of compression
[42e45]. Fig. 5.28C shows an open geothermal heating system, supplying both hot