System Description

The Hot Dry Rock (HDR) concept uses heat recovered from subsurface rocks to generate electricity. The system proposed for extracting heat from the rock and converting it to electricity is comprised of two distinct subsystems (see Figure 1) at very different stages of their technological evolution. The two subsystems are the power plant (on th e surface) and the HDR reservoir (deep beneath the surface), which are connected by deep wells. The wells and reservoir are thought of as a single system, often referred to as the well field system or reservoir system. The power plant system is largely identical to commercial binary hydrothermal electric plants. The technology for the reservoir

Electric Power

Electric Power

Figure 1. Hot dry rock electric power generation schematic.

system is much less mature. HDR reservoir creation and use has been demonstrated at experimental sites in the U.S. , Europe, and Japan, but not on a commercial scale.

The reservoir subsystem is developed by drilling wells into hot rock about 4 kilometers deep, and connecting the wells through hydraulic fracturing. Water, from a nearby fresh water well or other source, is pumped through one or mor e injection wells into the reservoir, where it is heated by contact with the hot rock, and then recovered through two or more production wells.

At the surface, the power plant subsystem converts the extracted heat to electricity using commercial binary power plant technology. First, the produced hot water passes through a heat exchanger, transferring heat to a working fluid in the power plant. The working fluid is characterized by a low boiling temperature; hydrocarbons such as iso-pentane, iso-

butane, etc. are typically used. The vaporized working fluid is expanded across a turbine to drive a generator an d produce electricity. The vaporized working fluid is then condensed in a cooling system and recirculated to the hea t exchanger. The hot water, upon exiting the heat exchanger, is injected back into the reservoir to collect additional heat.

The major components of a HDR system are described briefly below:

1. One, or more, hot dry rock reservoirs, created artificially by hydraulically fracturing a deep well drilled into hot, impermeable, crystalline basement rock. The hydraulic fracturing, achieved by pumping water into the well at high pressure, forces open tiny pre-existing fractures in the rock, creating a system or "cloud" o f fractures that extends for tens of meters around the well. The body of rock containing the fracture system is the reservoir of heat. The fracture system provides for the heat transport medium, water, to contact a large area of the rock surface in order to absorb the heat and bring it to the surface. More than one reservoir could supply hot water to a single power plant.

2. Deep wells for production and injection of water. The wells are drilled with conventional rotary drillin g technology similar to that used for drilling deep oil and gas wells. The total number of wells and the rati o of production wells to injection wells may vary. Experimental HDR systems to date have typically involved one injection well and one production well. The earliest commercial HDR systems will likely include a "triplet," two production wells for each injection well. A triplet of deep wells will support about 5 M W of power plant capacity, assuming adequate flow rates and fluid temperature. It is possible that other wel l configurations, such as a quadruplet (3 production wells per injection well) or a quintuplet (4 productio n wells per injection well) could be used. However, the cost effectiveness of using a quadruplet or quintuplet has not been established. Also, the ellipsoidal, rather than spherical, shape of the fracture pattern at Fenton Hill suggests that one production well on each side of the injection well, on the long axis of the reservoir , is the logical configuration. For these reasons, this analysis is limited to a ratio of two production wells per injection well, with earlier commercial systems limited to three wells total, and later systems using multipl e triplets of wells.

The original well, from which the fracture system is created, is used for injection. Two additional nearb y wells are drilled directionally to intersect the fracture system and are used as production wells. Operatio n of the system involves pumping water into the fracture system through the injection well, forcing it through the fracture system where it becomes heated, and recovering it through the production wells.

3. A system of microseismic instruments in shallow holes around the well that is being fractured. During th e fracturing operation, this system gathers seismic data, which is used to determine the extent and th e orientation of the hydraulically created fracture system. This information is then used to guide the drillin g of the production wells so that they intersect the fracture system at depth. Although the HDR system, once it is completed, can operate without it, the microseismic system is included here because it is an integral part of creating the HDR reservoir and because it may be left in place to gather additional information whic h could be useful later in the life of the HDR system. Note that the microseismic instruments are not depicted in Figure 1.

4. A shallow water well to provide water (or other source of fresh water).

5. Surface piping, or "gathering system," to transport water between the wells and power plant.

6. A binary power system to convert the heat in the water to electricity. This system is comprised of th e following major components:

a. One or more turbines connected to one or more electric generators.

b. A heat exchange vessel to transfer heat from the hot water to a secondary working fluid with a lo w boiling temperature.

c. A heat rejection system to transfer waste heat to the atmosphere and condense the vapor exiting the turbine. A wet, or dry, cooling system can be used. The capital cost of a wet cooling system is onl y marginally less expensive than for a dry cooling system. However, this cost advantage is largely offset by the higher operating cost of the wet cooling system. For this reason, and since HDR sites in the U.S. are likely to be in arid areas with limited water supplies, this technology characterization is limited t o a dry cooling system.

d. Injection pump(s) to circulate the water through the HDR reservoir.

e. Pumps to repressure the working fluid after it condenses and a vessel (not shown in Figure 1) for storing the working fluid.

f. Electrical controls and power conditioning equipment.

Additional information on binary systems can be found in the geothermal hydrothermal technology characterizatio n and in Reference [1].

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