Environmental Engineering Reference
In-Depth Information
rejected to the environment. Thus, the heat pump used as a refrigerator has lower
performance than when used as a heater. For the most efficient use of energy re-
sources, both the hot and cold sources of the heat pump should be used. For ex-
ample, the hot source for heating water as in the Eco-Cute hot water heater and the
cold source for refrigeration or cooling should be used simultaneously to maximize
the efficiency of the input energy.
5.3.7
Geothermal Energy Systems
In geothermal energy systems, heat from the Earth serves as the hot source ( Q h ) and
the environment serves as the cold source ( Q c ). A working fluid is used in a Rankine
cycle to generate electricity from geothermal and waste heat (ca. 100 °C) heat. In
principle, the process systems look similar to those in Fig. 5.6 with Q h being the
geothermal or waste heat hot source and Q c being the environmental temperature
(this could be that of the air, earth, river or sea). The working fluid can be CO 2 in
cases where domestic electrical energy is required, or the working fluid can be an
organic solvent for cases where industrial electrical energy is required, sometimes
in conjunction with petroleum production.
5.3.8
Ultra-supercritical Steam Generators
Ultra-supercritical (USC) steam cycles operate at much higher temperatures (ca.
700 °C) and pressures (ca. 30 MPa) than those of boiling-water Rankine cycles. The
advantage of USC steam cycles compared with boiling-water steam cycles is that
USC steam cycles can achieve much higher efficiencies (ca. 50 %) compared with
low-pressure Rankine cycles (ca. 30 %). Due to their higher efficiencies, USC steam
cycles have lower fuel (coal, biomass) requirements and generate lower amounts of
CO 2 compared with low-pressure Rankine cycles. In principle, the process system
for a USC steam cycle appears as Fig. 5.6 with Q h being supplied by coal, fuel oil or
biomass and Q c being supplied by river water or the sea. On a temperature-entropy
diagram, the process steps would be represented as shown in Fig. 5.8 .
In Fig. 5.8 , liquid water at 100 °C ( Point 1 ) is compressed to 30 MPa (Point 2).
Heating of the compressed liquid from Point 2 to 3 is by heat exchange with com-
bustion gases from a fuel source such as coal, petroleum or biomass. During the
heat exchange process, there is no phase change. Then from Point 3 to 4 , the stream
produces work through a high-pressure expander. The effluent stream is regenerated
by contact with exit gases from the supercritical steam generator from Point 4 to 5 .
Then, a low-pressure expander is used to obtain additional work from Point 5 to 6 .
Sea water or river water is used in the condensation step ( Point 6 to 1) that is nec-
essary to recycle the water. There are some practical considerations for the system
regarding materials of construction used at the high-temperature and high-pressure
conditions, corrosion, maintenance, and the optimum number of devices. However,
Search WWH ::




Custom Search