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The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as two units of thermal energy for each unit of electrical energy used, thus apparently contradicting the principle of energy conservation.

Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapor phase in a closed loop. The refrigerant, starting as a low-temperature, low-pressure vapor, enters a compressor driven by an electric motor. The refrigerant leaves the compressor as a hot, dense vapor and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body of air. Now the refrigerant, as a high-pressure, cooled liquid, confronts a flow restriction which causes the pressure to drop. As the pressure falls, the refrigerant expands and partially vaporizes, becoming chilled. It then passes through a second heat exchanger, the evaporator, which transfers heat from the air to the refrigerant, reducing the temperature of this second body of air. Of the two heat exchangers, one is located inside, and the other one outside the house, so each is in contact with a different body of air: room air and outside air, respectively.

The flow direction of refrigerant through a heat pump is controlled by valves. When the refrigerant flow is reversed, the heat exchangers switch function. This flow-reversal capability allows heat pumps either to heat or cool room air.

Now, if under certain conditions a heat pump puts out more thermal energy than it consumes in electrical energy, has the law of energy conservation been challenged? No, not even remotely: the additional input of thermal energy into the circulating refrigerant via the evaporator accounts for the difference in the energy equation.

Unfortunately there is one real problem. The heating capacity of a heat pump decreases as the outdoor temperature falls. The drop in capacity is caused by the lessening amount of refrigerant mass moved through the compressor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant being compressed, the less the thermal load it can transfer through the heat-pump cycle. The volume flow rate of refrigerant vapor through the single-speed rotary compressor used in heat pumps is approximately constant. But cold refrigerant vapor entering a compressor is at lower pressure than warmer vapor. Therefore, the mass of cold refrigerant — and thus the thermal energy it carries — is less than if the refrigerant vapor were warmer before compression.

Here, then, lies a genuine drawback of heat pumps: in extremely cold climates — where the most heat is needed — heat pumps are least able to supply enough heat.

1. The primary purpose of the text is to

[A] explain the differences in the working of a heat pump when the outdoor temperature changes.

[B] contrast the heating and the cooling modes of heat pumps.

[C] describe heat pumps, their use, and factors affecting their use.

[D] advocate the more widespread use of heat pumps.

2. The author resolves the question of whether heat pumps run counter to the principle of energy conservation by

[A] carefully qualifying the meaning of that principle.

[B] pointing out a factual effort in the statement that gives rise to this question.

[C] supplying additional relevant facts.

[D] denying the relevance of that principle to heat pumps.

3. It can be inferred from the text that, in the course of a heating season, the heating capacity of a heat pump is greatest when

[A] heating is least essential.

[B] electricity rates are lowest.

[C] its compressor runs the fastest.

[D] outdoor temperatures hold steady.

4. If the author’s assessment of the use of heat pumps (lines 1-4) is correct, which of the following best expresses the lesson that advertisers should learn from this case?

[A] Do not make exaggerated claims about the products you are trying to promote.

[B] Focus your advertising campaign on vague analogies and veiled implications instead of on facts.

[C] Do not use facts in your advertising that will strain the prospective client’s ability to believe.

[D] Do not assume in your advertising that the prospective clients know even the most elementary scientific principles.

5. The text suggests that heat pumps would be used more widely if

[A] they could also be used as air conditioners.

[B] they could be moved around to supply heat where it is most needed.

[C] their heat output could be thermostatically controlled.

[D] people appreciated the role of the evaporator in the energy equation.

[答案與考點解析]

1. 【答案】C

【考點解析】這是一道中心主旨題。本文的第二、三、四段描述了“heat pumps”的相關物理原理，第一、五、六段談到了影響“heat pumps”應用的原因。由此可見本題的正確選項應該是C。考生在解題時一定要對全文的整體結構有所認識，并將各段的主題句聯系起來加以理解。

2. 【答案】C

【考點解析】這是一道段落間關系題。通過題干中的“the question of whether heat pumps run counter to the principle of energy conservation”可迅速確定本題的答案信息來源應該在第二段，因為第一段就是本題的題干。通過仔細閱讀原文可發現本題的答案信息來源在第二、三、四段，在這三段中本文作者為解決相關問題給出了“relevant facts”(相關事實)。可見本題的正確選項應該是C。考生在解題時一定要注意段落之間的相互關系。

3. 【答案】A

【考點解析】這是一道審題定位與反推題。通過本題題干中的“heating season”可迅速將本題的答案信息來源確定在尾段，因為尾段中的“extremely cold climates”暗示出“heating season”。通過仔細閱讀和理解尾段并且進行相應的推導就可得出本題的正確選項A。考生在解題時首先要具備審題定位的能力，另外在解題時不能僅僅停留于字面含義，要多動腦子進行合理的推導。

4. 【答案】C

【考點解析】這是一道歸納推導題。題干把本題所涉及的問題確定在第一段，而本文作者對于該問題的解釋和說明確在第四段。通過對這兩段的綜合歸納推導，可得出本題的正確選項是C。由于作者在第四段的深刻解釋幫助解決了人們心中的疑惑，但是這兩段的深層含義暗示我們：人們在做廣告時要避免使用超出消費者可信度的信息。考生在解題時要注意段落之間的聯系，更要注意作者所要傳遞的深層含義。

5. 【答案】D

【考點解析】這是一道審題定位與段落間關系題。通過題干中的“heat pumps would be used more widely”可迅速確定本題的題干來自于首段，在首段中我們也可以得知阻礙“heat pumps”被大家廣泛接受的原因是“contradicting the principle of energy conservation”。在本文的第四段作者對第一段中所涉及的問題給出了合理的解釋，指出“heat pumps”并沒有“contradicting the principle of energy conservation”。問題出在人們對于“evaporator”缺乏了解。綜合這兩段的內容我們可得知：如果人們對“evaporator”有所認識，他們對“heat pumps”就不會心存疑慮，同時他們就會接受“heat pumps”，從而“heat pumps”就會得到廣泛的利用。可見本題的正確選項應該中D。考生在解題時一定要注意段落之間的聯系，更要注意反推即逆向思維的應用。

[參考譯文]

熱泵使用受到阻礙，主要是人們懷疑廣告上所宣稱的，熱泵能夠提供兩倍于其所消耗電能的熱能，這顯然與能量守恒定理相悖。

熱泵循環使用的是液體制冷劑，這個循環使得制冷劑在一個封閉環路內從液體到氣體交替變化。循環開始時，制冷劑以低溫低壓的蒸氣，進入一個電機驅動的壓縮機。出了壓縮機后，制冷劑變為高溫稠密的氣體，流經一個被稱為冷凝器的熱交換器，這個冷凝器把制冷劑的熱量傳送到一個氣團。制冷劑就變成了高壓冷卻的液態，然后經過一個節流器，節流器使其壓力下降。壓力下降時，制冷劑就會膨脹并且使部分液體氣化，制冷劑變冷。然后經過第二個熱交換器即蒸發器，蒸發器把熱量從空氣中傳到制冷劑中，使得第二個氣團溫度降低。這兩個熱交換器，一個在室內工作，另外一個位于室外，所以每個熱交換器接觸各自不同的氣團：室內空氣和室外空氣。

制冷劑在熱泵內的流動方向受閥門控制。當制冷劑逆向流動時，兩個交換器就交換它們的功能。這個逆向流動的能力使得熱泵對室內空氣加溫或冷卻。

現在，如果在某些條件下一個熱泵輸出的熱能多于其消耗的電能，則能量守恒法則受到挑戰了么?不，絲毫沒有：通過蒸發器進入了制冷劑循環的額外熱能，可以說明能量平衡上的差異。

不幸的是，這里有一個現實的問題。熱泵的加熱能力隨著室外溫度的下降而減少，其減少是由于在固定時間內流經壓縮機的制冷劑數量上的減少而造成的。熱容量和制冷劑質量流動速率成比例：被壓縮的制冷劑越少，加載到熱機循環中傳輸的熱量越少。在使用單速旋轉式壓縮機的熱泵中，制冷劑氣體的體積流動速率近似一個常數。但進入壓縮機的制冷劑氣體，其冷卻的氣體壓力比熱氣壓力要小。因此，冷的制冷劑氣體質量，也就是它攜帶的熱能小于在壓縮之前比較熱的制冷劑氣體的質量。

這樣，那么熱泵存在的真正障礙在于：在極端寒冷氣候地區，那里最需要熱量，熱泵卻最不能夠提供充足的熱量。

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（責任編輯：rhj）

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