Page 262 - Low Temperature Energy Systems with Applications of Renewable Energy

Geothermal energy in combined heat and power systems 247

involved, the amount of exergy exchanged is likewise very low. From Table 6.3,it may be
seen that about 32 kW of heat exergy is released by the distributed hot water altogether. If
the hot water tank raises the domestic water temperature from, say, 25 Cto60 C, it would

gain about 13 kW and the transfer would be about 52% efficient, ignoring any other uses
outside the building.

6.6 Case studies

There is widespread use of geothermal CHP systems across the world [9]; only two
plants have been selected for discussion. The next two sections present case studies
based on the experiences of the author of this chapter (RDP). The first one is a
medium-size geothermal flash-steam plant that supplied separated brine to a hot water
heating plant for distribution to a municipality, and the other is a small binary plant
combined with a heating system for a college campus. The case studies draw upon
the author’s earlier works on these two systems [10,11].

6.6.1 Kakkonda-Shizukuishi, Honshu, Japan

The Kakkonda-Shizukuishi CHP facility was built in the late 1980s and at its peak
generated 50 MW of electrical power and supplied 1050 t/h (292 kg/s) of hot water
to industrial, commercial and residential end users. It was the largest project of its
type in Japan at the time. Kakkonda Unit 1 began operating as a stand-alone,
single-flash power plant in 1978. The plant is a fairly standard design with five produc-
tion and reinjection well pads, with a set of three cyclone separators at Pad B, just
below the power station. Steam was sent from the well pads to pad B and on to the
power house via a network of pipes; the separated brine was reinjected back to the
reservoir via wells on the pads. The available surface area for construction is limited
owing to the rugged mountainous terrain on both sides of the Kakkonda River that
flows past the plant site. This necessitated directional drilling from a few well pads,
situated relatively close to the river, to minimize interference among production and
injection wells.
Figure 6.20 shows the power house and well pad B, one of the closest to the plant.
After several years of successful operation, it was decided to add a hot water produc-
tion facility (HWPF) to complement the electrical power by capturing some of the en-
ergy from the separated brine that had been completely reinjected. The town of
Shizukuishi, whose center lies some 17 km southeast of the power plant, would be
the beneficiary of the heat. Given the rugged terrain at the plant site, the HWPF was
sited about 1.5 km away from the power station in the direction of Shizukuishi; see
map Fig. 6.21.
Owing to difficult topography between the power plant and the town, the hot water
pipeline passes over bridges and through tunnels constructed for this purpose;
Fig. 6.22 shows the path in schematic form. Figure 6.23 shows the distribution pipe-
lines within Shizukuishi town. The pipelines take advantage of gravity as the town lies
about 420 m lower than the HWPF.