首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as
The use of heat pumps has been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as
admin
2013-01-12
41
问题
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 vapour 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 vapour 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 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 singlespeed 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 if the refrigerant vapor were warmer before compression.
Here, then, lies a genuine drawback of heat pumps: in extremely cold climates heat is needed—heat pumps are least able to supply enough heat. (441 words)
What conclusion can we make based on the whole passage?
选项
A、Heat pumps are not efficient in supplying heat in coldness.
B、Heat pumps can find applications everywhere.
C、Heat pumps are of no use at all.
D、The inner structure of a heat pump is so complex that it has already been out of date.
答案
A
解析
文章最后的结论是“热泵的确存在着问题,即在极度寒冷的天气里,人们需要热能,热泵所提供的热 能是远远不够的。”
转载请注明原文地址:https://kaotiyun.com/show/Xa9O777K
0
考博英语
相关试题推荐
AnewcatastrophefacesAfghanistan.TheAmericanbombingcampaignisconspiringwithyearsofcivilconflictanddroughttocre
Ididn’tsayanythinglikethatatall.Youare______purposelymyideastoproveyourpoint.
Ihopeyouarenot______thatIobtainedthismoneydishonestly.
Severalguestswerewailingonthe______forthefrontdoortoopen.
Thefoodyoueatdoesmorethanprovideenergy.Itcanhaveadramaticeffectonyourbody’sabilitytofightoffheartdisease,
AstudybyscientistsinFinlandhasfoundthatmobilephoneradiationcancausechangesinhumancellsthatmightaffectthebr
AnAmericanliterarycritic,humorousjournalist,essayist,whosecomicskepticismabouthumanprogress,expressedwithpenetrat
Recentlyscientistshavebeen【1】andperfectingothersourcesofenergy:nuclearoratomicpower,solar(sun)power,andsyntheti
ItispossibleforstudentstoobtainadvanceddegreesinEnglishwhileknowinglittleornothingabouttraditionalscholarlyme
随机试题
简述可摘局部义齿就位后应检查的内容。
简述血浆晶体渗透压及血浆胶体渗透压的基本概念及功能。
患儿,7岁。近1周来,右下后牙食物嵌塞。检查:右下第一乳磨牙远中邻面深龋洞。探敏感,叩(一),无松动,龈正常。右下第二乳磨牙大面积龋坏,探及髓腔,探痛(一),叩(+)。颊侧瘘管溢脓,工度松动,左下第一、二乳磨牙残冠,Ⅰ度松动,叩(+),龈红肿。双侧六龄牙萌
二尖瓣关闭不全时心脏的叩诊特点是
保证期间,债权人许可债务人转让债务的,有关保证人的权利和义务的表述哪一项是正确的?
根据《企业会计准则第8号——资产减值》的规定,资产的可收回金额应当根据下列哪两个中的较高者确定()。
阅读以下文字,完成61~65题。我们生活在一个复杂的世界,因此不得不努力将其简化。我们把周围的人归类为朋友或敌人,将他们的动机分成善意或恶意,并将事件的复杂根源归结为直接的原因。这些捷径帮助我们游弋于社会存在的复杂性之中,协助我们对自己和他人的行
社会思潮是一面镜子,对社会思潮进行观察认识,可以帮助我们从社会的精神层面更加深入地了解当代中国社会的历史发展与变迁。中国在经济现代化的同时,也在经历一个文化现代化的过程。譬如,自由主义、民族主义和“新左派”等思潮无不突出而集中地反映了当代中国社会发展中的矛
设有定义:intk=1,m=2;floatf=7;,则以下选项中错误的表达式是
Owlsareairborneinvaders-powerfulinflight,intimidatinginreposeandperchinginnumbersfarbeyondanythinganyoneremembe
最新回复
(
0
)