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  3. The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial

The energy contained in rock within the earth’s crust represents a nearly unlimited energy source, but until recently commercial

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问题     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 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 the recent energy crisis, new concepts for creating HDE recovery systems—which involve drilling holes and connecting them to artificial reservoirs placed deep within the crust—are being developed. (46) In all attempts to retrieve energy from HED’s reservoir 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.
    (47) The HDR resource base is generally defined to include 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; 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.
    (48) Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tapable 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. (49) 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 percept is recoverable, we find a total that is comparable to the estimated resource base of all the coal remaining in the United States. (50) 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.
选项
答案剩下的问题是如何权衡更深更热而费用更大的热源井与较浅较冷而费用较小的热源井同最终产品(电力和热能，或者是电力或热能)的价值对比之间的利害得失关系。
解析     句子框架是The remaining problem is to balance the economies of…and…against the value of the final product，electricity and/or heat。此句是一个不定式作表语的长单句，介词of的宾语应为and连接的两个对比成分与against介词所带宾语分别对比之后的结果。economies原意是“经济学”，这里译为“利害得失”。
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