In the other type, the working fluid remains in the gaseous state. These
engines operate on various cycles, including the Stirling and Brayton cycles.
For relatively low thermal energy input temperatures (less than 400 deg. F),
Rankine cycle engines are superior in performance to gas cycle engines. At
higher temperatures, gas cycle engines equal or better the performance of
Rankine cycle engines.
Relatively low temperatures are attainable with state-of-the-art thermal
solar collectors, so the heat engine-vapor compression development projects
involve Rankine cycle engines.
In a Rankine cycle engine, fluid in the liquid state is pumped into a boiler
where it is evaporated and possibly superheated by thermal energy. The vapor
generated in the boiler is then expanded through a device such as a turbine,
a piston-cylinder (reciprocating) expander, or a rotary vane expander. The
expansion process lowers the temperature and pressure of the vapor, and
effects a conversion of thermal energy into shaft work. The fluid leaves the
expander either in the vapor phase or as a liquid-vapor mixture and flows
into a condenser, where it returns to the liquid phase by giving the energy
of condensation to cooling water or ambient air. This liquid is then pumped
into the boiler, and the cycle is repeated.
In some systems under development, the same working fluid is used in both the
Rankine engine and the vapor compression chiller, which permits the use of
common condenser and the elimination of special seals to maintain fluid
separation in the expander-compressor unit (Scholten and Curran, 1979).
These systems have areas that need development in matching the solar heat
engine with the mechanical compressor units of the cooling equipment. Since
most compressors are designed for certain speed and torque inputs, the
varying operation of a solar heat engine will probably reduce the overall COP
of the unit. Also the solar heat engine is at high efficiency at high
storage tank temperatures whereas the solar collectors are at low efficiency
which will also affect the COP of the system. These systems are designed for
large cooling load applications.
2.7.3 Desiccant cooling (Scholten and Curran, 1979). The Rankine engine-
vapor compression and the absorption cooling units operate on the basis of
closed cycles-fixed amounts of working fluid are circulated within sealed
equipment; the working fluids do not come in contact with the building air.
Desiccant cooling systems, on the other hand, may be designed for open-cycle
operation, since the only circulating fluids involved are air and water. The
basic concept is to dehumidify air with a desiccant, evaporatively cool the
dehumidified air, and regenerate the desiccant with solar-derived thermal
Two basic open-cycle arrangements are feasible: the ventilation mode and the
recirculation mode. In the ventilation mode, fresh air is continually intro-
duced into the conditioned space. In the recirculation mode, exhaust air
from the conditioned space is reconditioned and returned to the space.
Figure 2-25 illustrates a ventilation system in which a solid desiccant
material mounted on a slowly rotating wheel provides the basis for obtaining
a cooling effect.