Page 249 - Low Temperature Energy Systems with Applications of Renewable Energy

236 Low-Temperature Energy Systems with Applications of Renewable Energy

Fig. 6.15 ORC processes in pressure-enthalpy (P-h) and temperature-entropy (T-s) coordinates;
not to scale [4].

the reservoir by means of a well pump, either mechanical, shaft driven or electrically
powered submersible, reaching the surface at state A (Fig. 6.14). It passes through the
evaporator where it brings the working fluid to a saturated or slightly superheated va-
por condition, then through the preheater where it supplies the sensible heat to raise the
working fluid to a saturated liquid state, called its bubble point. From that point, the
geofluid is available for direct heat usage prior to being reinjected. The temperature
of the geofluid here is typically significantly lower than in the case of steam plants,
but can still be useful for space heating or spas, among other applications.
ORCs can be divided into two categories depending on the pressure in the heat ex-
changers: subcritical and supercritical. Figs. 6.14 and 6.15 are drawn for the subcritical
case, i.e., processes 5-6-1 are at a pressure lower than the critical pressure; note the
critical point lies at the top of the saturation curves. There is a fundamental difference
between these two categories: a supercritical cycle allows for a much closer match be-
tween the temperature of the geofluid and the working fluid in the PH and E heat ex-
changers, and results in a more efficient transfer of heat, i.e., a smaller loss of exergy.
This is evident in Fig. 6.16 which shows the heat transfer processes for the two
cases in a temperature-heat transfer diagram. It is schematic but illustrates that the
mean effective temperature difference (METD) is much larger for the subcritical
case because of the effect of the pinch-point, DT pp , which occurs along a much
smoother curve in the supercritical case owing to the continuous shape of the isobar.
The shaded areas are proportional to the METD. The narrower gap between the heating
and cooling curves means there is less irreversibility and less loss of exergy during the
heat transfer. For this particular example, the working fluid is raised to a higher tem-
perature and the geofluid is cooler at the discharge (state C) in the supercritical case,
but the latter observation is not a general conclusion. It follows that a larger heat
exchanger is need to transfer the same amount of heat when the METD is smaller,
which implies higher cost. Thus, thermodynamic efficiency comes at a higher capital
cost of equipment.
ORCs may be used instead of flash-steam units to form a CHP system, as shown in
Fig. 6.17. Bypass pipelines should be used to allow the hot water plant to operate when
the power plant is unavailable or when the demands for heat and electricity vary.