20. Which of the following techniques is used by the author in developing the argument in the passage?
(A) Giving a cliché a new meaning
(B) Pointedly refusing to define key terms
(C) Alternately presenting generalities and concrete details
(D) Concluding the passage with a restatement of the first point made in the passage
(E) Juxtaposing statements of what is not the case and statements of what is the case
The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial retrieval has been limited to underground hot water and/or steam recovery systems. These systems have been developed in areas of recent volcanic activity, where high rates of heat flow cause visible eruption of water in the form of (in the form of: 以...的形式) geysers and hot springs. In other areas, however, hot rock also exists near the surface but there is insufficient water present to produce eruptive phenomena. Thus a potential hot dry rock (HDR) reservoir exists whenever the amount of spontaneously produced geothermal fluid has been judged inadequate for existing commercial systems.
As a result of recent energy crisis, new concepts for creating HDR recovery systems—which involve drilling holes and connecting them to artificial reservoirs placed deep within the crust—are being developed. In all attempts to retrieve energy from HDR’s, artificial stimulation will be required to create either sufficient permeability or bounded flow paths to facilitate the removal of heat by circulation of a fluid over the surface of the rock.
The HDR resource base is generally defined to included crustal rock that is hotter than 150℃, is at depths less than ten kilometers, and can be drilled with presently available equipment. Although wells deeper than ten kilometers are technically feasible, prevailing economic factors will obviously determine the commercial feasibility of wells at such depths. Rock temperatures as low as 100℃ may be useful for space heating (heating of spaces especially for human comfort by any means (as fuel, electricity, or solar radiation) with the heater either within the space or external to it); however, for producing electricity, temperatures greater than 200℃ are desirable.
The geothermal gradient, which specifically determines the depth of drilling required to reach a desired temperature, is a major factor in the recoverability of geothermal resources. Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tappable high-temperature gradients are distributed all across the United States. (There are many areas, however, for which no temperature gradient records exist.)
Indications are that the HDR resource base is very large. If an average geothermal temperature gradient of 22℃ per kilometer of depth is used, a staggering 13,000,000 quadrillion B.T.U.’s of total energy are calculated to be contained in crustal rock to a ten-kilometer depth in the United States. If we conservatively estimate that only about 0.2 percent is recoverable, we find a total of all the coal remaining in the United States. The remaining problem is to balance the economics of deeper, hotter, more costly wells and shallower, cooler, less expensive wells against the value of the final product, electricity and/or heat.
21. The primary purpose of the passage is to
(A) alert readers to the existence of HDR’s as an available energy source
(B) document the challengers that have been surmounted in the effort to recover energy from HDR’s
(C) warn the users of coal and oil that HDR’s are not an economically feasible alternative
(D) encourage the use of new techniques for the recovery of energy from underground hot water and steam
(E) urge consumers to demand quicker development of HDR resources for the production of energy
22. The passage would be most likely to appear in a
(A) petrological research report focused on the history of temperature-depth records in the United States
(B) congressional report urging the conservation of oil and natural gas reserves in the United States
(C) technical journal article concerned with the recoverability of newly identified energy sources
(D) consumer report describing the extent and accessibility of remaining coal resources
(E) pamphlet designed to introduce homeowners to the advantages of HDR space-heating systems