Page 211 - Geothermal Energy Renewable Energy and The Environment

198 Geothermal Energy: Renewable Energy and the Environment

1.30
y = 1.1067 + 0.0000292 • x (R = 0.40)
1.25

1.20
Temperature ratio 1.15

1.10

1.05

1.00
0 500 1000 1500
Time (Hrs.)

FIGUre 10.14 Variation in the ratio of borefield supply temperature to return temperature to the borefield
for the time period from November 4, 2005 through January 4, 2006. The light-gray line is a least squares fit
to the data. The equation for the line is shown in the figure, as is the goodness of fit.

synopsIs

Ground source heat pump systems take advantage of the low temperature geothermal resource that
exists in the shallow subsurface (less than 200 m depth). The constant flux of heat from the interior
of the Earth plus the influx of solar energy to soils provides a reliable thermal reservoir from which
heat can be extracted. High efficiency heat pumps that are coupled to this reservoir can move the
thermal energy from the Earth to buildings by use of simple heat pumps. The heat pump technology
currently available allows COP values in the range of 3 to 5, which makes them the most energy
efficient means for HVAC purposes. Sizing such systems is a straightforward and well-developed
practice, but relies on the availability of high quality information regarding soils and properties of
the building to assure that a functioning system reliable meets the demand placed on it.

Problems
10.1 Describe how a ground source heat pump works. What determines its efficiency?
10.2 What does COP measure? What variables go into computing COP?
10.3 How might you change the performance of a heat pump so that its COP went from
3.5 to 5.0?
10.4 At 25°C, how much heat is available, per degree temperature drop, from a soil com-
posed of equal parts quartz, alkali feldspar, and calcite, with a water-filled porosity
of 15%?
10.5 For a heating system employing a heat pump with a COP of 3.8 for a building heating
load of 15 kW, calculate the length of the heating loop needed. Assume the soil proper-
ties are those in Problem 10.4, that the thermal conductivity of the pipe is the same as
the soil, the fractional heating time is 0.5, that the minimum soil temperature is 10°C,
and the heat pump fluid minimum temperature is 3°C.
10.6 From the Weaverville case study, explain, hour by hour from Figure 10.12, the factors
that are controlling the temperature for the return from the room heat pump.
10.7 What would be the advantages and disadvantages of maintaining the thermostat at the
same temperature 24 hours a day, if the pattern in Figure 10.12 is being considered?