ENERGY GENERATION BY HOT ROCK ENERGY (HDR) TECHNOLOGY
Posted June 23, 2008on:
Hot Rock Energy is a vast, environmentally friendly, economically attractive energy source.
The concept is very simple.
Water is injected into a borehole and circulated through a “heat exchanger” of hot cracked rock several kilometres below the surface. The water is heated through contact with the rock and is then returned to the surface through another borehole where it is used to generate electricity. The water is then re-injected into the first borehole to be reheated and used again.
Hot Dry Rock Geothermal Energy
HDR geothermal energy relies on existing technologies and engineering processes, and is the only known source of renewable energy with a capacity to carry large base loads. The concept behind HDR geothermal energy is relatively simple. Heat is generated by special high heat producing granites located 3km or more below the Earth’s surface. The heat inside these granites is trapped by overlying rocks which act as an insulating blanket. The heat is extracted from these granites by circulating water through them in an engineered, artificial reservoir or underground heat exchanger.
HDR geothermal energy relies on existing technologies and engineering processes such as drilling and hydraulic fracturing, techniques established by the oil and gas industry. Standard geothermal power stations convert the extracted heat into electricity. HDR geothermal energy is environmentally clean and does not produce greenhouse gases.
It has been classified as renewable by National and International authorities.
The Hot Rock Energy system works with two closed circulation loops
The subsurface loop
This loop circulates water down an injection borehole where it passes through the
underground “heat exchanger” and is heated. The superheated water is then recovered by one or more production boreholes which return it under pressure to the surface. By keeping the water under pressure and preventing it turning to steam, any materials dissolved from the underground rock mass (such as silica or carbonates) are kept in solution and can be returned to the ground.
At the surface, the superheated water is passed through a metal heat exchanger where most of the heat is removed. The now cooled water is then returned to the injection borehole where it is sent down again to recover more heat.
The power station loop
At the surface a second closed loop fluid system is used to transfer the heat into the power station and generate the electricity in a turbine. The fluid used in the power station loop can be water, but more usually a lower boiling point fluid is used. Organic fluids such as refrigerants and iso-pentane are often used.
The HDR Energy Extration Process
All modern HDR development work is based on the relatively simple concept described in a US Patent issued to Los Alamos when HDR technology was more theory than reality. A well is first drilled into hot, crystalline rock. Water is then injected at pressures high enough to open the natural joints in the rock, thereby creating an engineered geothermal reservoir.
The reservoir consists of a relatively small amount of water dispersed in a very large volume of hot rock. The relative dimensions and orientation of the reservoir are determined by the local geologic and stress conditions, while its ultimate volume is a function of the duration of the hydraulic fracturing operation and the fracturing pressures applied.
Seismic techniques are used to follow the growth of the reservoir, to assess its location, and to determine its approximate dimensions. Using the seismic data as a guide, one or more production wells are subsequently drilled into the engineered
reservoir at some distance from the first well. In a properly engineered HDR reservoir, there are a number of fluid-flow pathways between the injection and production wellbores.
Operation of an HDR heat mine is extremely simple. A high-pressure injection pump
provides the sole motive force to circulate water through the engineered reservoir and deliver it to a power plant on the surface. The hydraulic pressure applied via the injection pump also serves to keep the joints within the reservoir propped open. The operating parameters applied to the injection pump thus greatly affect both the flow rate through the reservoir and its instantaneous fluid capacity. By using a Combination of injection and production control measures, an almost limitless variety
of operating scenarios may be employed to mine the heat.
The Major Components of a HDR System
1. One, or more, hot dry rock reservoirs are 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” of 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 are meant for production and injection of water. The wells are drilled with conventional rotary drilling technology similar to that used for drilling deep oil and gas wells. The total number of wells and the ratio 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 MW of power plant capacity, assuming adequate flow rates and fluid temperature. It is possible that other well configurations, such as a
quadruplet (3 production wells per injection well) or a quintuplet (4 production 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 multiple triplets of wells. The original well, from which the fracture system is created, is
used for injection. Two additional nearby wells are drilled directionally to intersect the fracture system and are used as production wells. Operation 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 is used. During the fracturing operation, this system gathers seismic data, which is used to determine the extent and the orientation of the hydraulically created fracture system. This information is then used to guide the drilling 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 itis an integral part of creating the HDR reservoir and because it may be left in place to gather additional information which could be useful later in
the life of the HDR system.
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 is used to convert the heat in the water to electricity. This system is comprised of the 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 low 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 only 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 to a dry cooling system.
d. Injection pump(s) circulate the water through the HDR reservoir.
e. Pumps repressure the working fluid after it condenses and a vessel storing the working fluid.
f. Electrical controls and power conditioning equipment.
Development of an Engineered Artificial Reservoir
Granites have an internal fabric of cooling joints and fractures, a result of cooling down from a melt (like molten glass) to a solid that we see today. Developing an underground heat exchanger involves increasing the hydraulic pressure at the bottom of a deep drill hole (approx. 4- 5km) until the existing fractures and joints are slightly opened.