Title: Absorption refrigeration system as an integrated condenser cooling unit in a geothermal power plant

Abstract

Geothermal energy is promising energy for any heat driven applications whether involving direct or
indirect utilization processes. A separation process of a geothermal fluid mixture is mostly needed
for indirect geothermal utilization, especially in power generation cycles. The separation process
disposes of the liquid form of low grade thermal energy which could be utilized further for other
direct and indirect utilizations such as a power plant bottoming unit, heating and cooling purposes
or other heat driven processes, depending on how much of the available energy remains.
As the steam condensation process is one of the main keys for achieving high power generation
efficiency, the temperature level of the condenser is very crucial. Condenser temperature regulates
the lowest pressure that can be applied to the condenser, not counting the presence of
non-condensable gases from geothermal fluids. Simulations for various cooling water temperatures
indicate that the change in the condenser’s pressure from 11.178 down to 8.651 kPa yields extra
power of about 45 and 33 kW for single flash and double flash systems, respectively. In some cases,
the preferred condenser pressure is not achievable due to the environmental factor, i.e. high
ambient temperatures in tropical countries. Hence, the environment cannot serve plant systems with
such a low temperature of the cooling water. Based on this fact, an artificial cooling system such
as an absorption refrigeration system could be built in order to produce the preferred temperature
of the cooling water. Heated cooling water from the outlet of a plant’s condenser is first cooled
by a regular cooling tower and then cooled again by the absorption system to produce a lower
temperature of the cooling water.
An absorption refrigeration system (ARS) as a heat driven refrigeration system could be powered
using geothermal brine water as its heat source. Single effect absorption refrigeration systems of
two well-known refrigerant-absorbent pairs - Water-Lithium Bromide pair and Ammonia-Water pair -
are modelled. Fed by a similar heat source, and providing the cooling load needed by the power
plant condensing system, two main aspects are observed: the Coefficient of Performance and the area
of heat exchangers needed to operate the absorption system. Two different scenarios are set for
this integrated power and refrigeration system: fixed mass flow for both power plant and
refrigeration system, and a dynamic mass flow scenario. In the dynamic mass flow scenario, some
quantity of mass flow which was previously fed to the power plant system can be freely switched to
the refrigeration system in order to accommodate such a refrigeration load.
Several combinations of temperature differences between the inlet and the outlet of an ARS
evaporator and the evaporator’s temperature were simulated to determine the optimum combination for
the given heat resource. Other parameters, for example the weak-strong solution concentration
difference, were also selected so that the absorption refrigeration system would work in high
refrigeration efficiency
with the least heat exchanger area.