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Toprak kaynaklı bir ısı pompası tesisinin tasarımı ve optimizasyonu

Design of ground coupled heat pump systems

  1. Tez No: 14368
  2. Yazar: HALİL ATAMAN
  3. Danışmanlar: DOÇ.DR. SALİM ÖZÇELEBİ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 141

Özet

ÖZET Son yıllarda dünyamızın yaşadığı ekonomik krizler ve yenilenemeyen enerji kaynaklarının tükenmeye yüz tutması, enrjinin ekonomik biçimde kullanımını zorunluluk haline getirmiştir. Isı pompalarının herhangibir alanı ısıtmak için tükettiği enerjinin daha fazlasını ısıtmak maksadıyla kullanması; birbaşka deyişle birincil enerji kaynağını diğer alternatif ısıtma sistemlerinden daha yüksek verimde kullanması, önemlerini arttırmıştır. Enerji fiyatlarının artması sonucunda, kullanım maliyetlerinin, alternatif ısıtma sistemleri yanında çok düşük kalması, ilk yatırım maliyetlerinin yüksek olması dezavantajını ortadan kaldırma aşamasına gelmiştir. Isı pompalarında kaynak olarak toprağın kullanımı, toprağa gömülen ısı değiştiricilerinin maliyetlerinden ötürü, kaynak olarak havayı kullanan ısı pompalarına göre daha yüksek yatırım maliyetleri gerektirmektedirler. Ancak toprak kaynaklı ısı pompalarının ısıtma tesir katsayılarının daha yüksek olması işletim maliyetlerini azaltmaktadır. Toprak ısı değiştiricilerinin maliyetlerinin yüksek olması, boyut landırılm a larının çok iyi yapılmasını gerektirir. Bunun içinse toprak özelliklerinin, gerek hava gerekse toprak sıcaklık değerlerinin ve ısı pompası karakteristiklerinin çok iyi bilinmesi gerekir. Ayrıca boyutlandırma sonunda ekonomik analizin çok iyi yapılması gerekir. Bu çalışmada, İstanbul Göztepe 'de inşa edilen bir konutun, toprak kaynaklı ısı pompası ile ısıtılması ele alınmış, konutun ısı kaybı derece gün yöntemiyle hesaplandıktan sonra toprak ısı değiştiricisi, Bin yöntemi kullanılarak boyutlandırılmıştır. Toprak ısı değiştiricileri boyut landırı lırken, topraktaki sıcaklık değişimi, Kelvin Çizgisel Kaynak Teorisi ile ayna görüntü yöntemi birlikte dikkate alınarak hesaplanmıştır. Bu yöntemin kullanılması için gerekli toprak direncinin hesabındaysa tüm boruların birbirlerine olan ısıl etkileşimi gözönüne alınmıştır. VII

Özet (Çeviri)

SUMMARY DESIGN OF GROUND COUPLED HEAT PUMP SYSTEMS Ground coupled heat pumps are systems utilizing ground heat exchanger designed to use the earth as a heat source and/ or sink. Ground coupling device is usually a long piece or several pieces of plastic pipe, buried horizontally or vertically in the ground. A fluid such as water or a brine, is circulated through the pipe, trans fer i rng thermal energy to or from the water to refrigerant heat exchanger. Ground coupling offers three significant advantages over use of air as a source or sink. 1. The ground temperature is more favorable than the air temperature. 2. The liquid-refrigerant exchanger permits a closer temperature approach than an air-refrigerant exchanger. 3. There is no concern with frost buildup or removal. The most significant point to understand the thermal behavior of ground coupled heat pump systems is that soil properties, and most importantly the thermal conductivity, may not be well known, and may vary with time. These soil properties vary with the heat pump operating time as well. Another significant factor regarding the thermal behavior of the ground coupling device is that the average temperature of the circulating fluid, continually increases in the cooling mode and comtinually decreases in the heating mode as the system continues to operate. The main concern in the design of the ground coupling device is that the circulating fluid temperatures obtained in actual operation be kept within some limits to prevent exessive wear or damage to the heat pump and to ensure efficient operation. The ground-exchanger performance is dependent upon the load placed on it by the pump. The temperature of the water returning to the heat pump is, in turn, dependent upon the performance of the ground exchanger. This return VIIIwater temperature controls both the capacity and coefficient of performance of the heat pump, which for a given building load determines the run time and cycling of the heat pump. Thus these components of the heat pump components of the system interact to affect heat pump. The temperature at which the ground exchanger water returns to the heat pump, the entering water temperature depends upon the frequency of the cycling and the past history of load, as well as upon the heat tranfer capability of the heat exchanger. This return water temperature, in turn, fixes the capacity of the heat pump. Thus there is a feed back effect, and this requires that some initial assumptions Csuch as frequency of cycling and entering water temperature) be made. The Design Sequence will typically follow the order given below. Design Sequence 1. Determine local design conditions Cclimatic and soil thermal characteristics). 2. Determine the building heating design loads and estimate monthly energy requirements. 3. Make a preleminary selection of a ground-coupling system Cvertical or horizontal). 4. Select the heat pump or heat pumps that will meet the demand calculated in step C2) and using a first estimate of average entering water temperatures determine coefficient of performance and capacities for the heat pump. 5. Using estimated capacities of the heat pump determine the run fraction for the prior month or months. 6. From manufacturers specifications select the design conditions to be imposed on the heat pump. This will include maximum and minimum return water temperatures. 7. Design the ground exchanger, fixing the pipe material and diameters, the layout of the pipe, and depths of the burial. 8. Calculate the thermal resistance of the ground exchanger. 9. Determine the lengths of the pipe required. The results may require returning to step C3) and IXsubsequent revision of the design. No sigle factor is more important to the successful operation of a ground-coupled heat pump system than the rate of heat transfer that determines how closely the circulating fluid, returning to the heat pump, can approach the local far field ground temperature. To simplify predictions this heat transfer is conveniently described in terms of the thermal resistance between the circulating fluid in the buried pipe and the undisturbed soil. This concept of the thermal resistance is a simplification, and describes the behavior of the entire subground system -the fluid, the coupling device and the soil surrounding that device. Because this resistance involves the make up the pipe sistem as well as the characteristics of the soil it is not a property of the soil only. A better description than soil resistance would be“the apparent thermal resistance between the circulating fluid and the undisturbed ground”. It might be called“field resistance”. This field resistance in the soil changes with time, even though all soil properties might be constant. Kelvin Line Source theory can be used to estimate the change in temperature of a buried pipe in which heat being absorbed or rejected. Q* 2 n k. ICx) t This equation gives a good estimate of the earth temperature at the buried pipe outer radius Cr), at a sipecified depth, and day of the year, for a known heat flow rate CQ*> and undisturbed soil temperature at the specified depth. The specific assumptions used in the Equation are:1. Soil properties are uniform and constant i.e., soil thermal conductivity, density and specific heat remain constant for all depths and time, 2. The heat flow rate per unit of pipe length is constant over the time period, and 3. The heat source is considered to be a line source, i.e., very long of and extremly small diameter. Heat pumps normally cycle on and off and do not provide a constant load on the ground exchangers. To use this method the heat rate must be averaged over the selected time period. In the Kelvin Line Source equation 2 n kt ICx) l R is the sytem's resistance to heat flow in the soil and is dependent upon the system configuration, operating time soil type and moisture content. When we consider t as the fluid temperature we must add pipe resistance CR. } in the equation as well. D t ~ t q/L = RL + R b t Where q - Q' L The absorbed heat rate per unit length of ground heat exchangerCq /L) is a function of the building load, the heat pump coefficient of performance CITK) and the ground exchanger length CD. During the heating season, the rate of heat to be absorbed is: XIITK - 1 q = q ' L TTK J b ' ITK The coefficient of performance CITK) is function of the ground exchanger fluid return temperature. The soil resistance value given in Kelvin line source equation is for continious heat absorption. For cyclic systems a run fraction multiplier f can be defined which averages the heat rate cyclic input over the time period under consideration. The run fraction is then defined as the ratio of the heat pump run hours devided by the time period. Using line source equation and substituting results in a design expression relating the above variables, the length of earth coil required can be expressed ITK L = q C R + R > b ITK *> i At At represents designer chosen temperature difference from the cyclic ground temperature If the chosen heat pump capacity is different from the building load, the heat pump capacity must be subsitituted instead of the building load. The Kelvin line source method assumes that the source is in an infinite medium. The method gives the temperature distribution with in time the soil around a pipe with acceptable accuracy when the pipe is very far away from the ground surface or from any other pipes that may also be heating. For a long sigle vertical ground coupling device the nethod requires minor corrections due to ground source effects. For horizontal systems, however, the nearby pipe requires that the line source method be modified. The mirror image method is used to approximate the effect of nearby influences. XIIThere are two assumptions for the surface coditions. The fist is adiabatic surface and the second is isothermal surface. Assuming that the line source is at a distance from an adiabatic surface, that is a surface across which there is no heat flow is to accept the line source is in a semi-infinite medium instead of the infinite medium assumed for the line source method. The solution for this case is obtained by imagining that an image source of the same strength as the original source exists in a semi-infinite medium and is at an equal distance and on the other side of the adiabatic source. The two systems together then consist of two parallel line source of equal strength in an infinite medium with an adiabatic surface at an equal distance from each source. The temperature distribution at any time can be obtained by solving for the temperature around each source assuming it is in an infinite medium and ignoring the other source. The two solutions are then added to give the temperature distribution for the single pipe near an adiabatic surface. This method could be used to determine the temperature distribution and the soil thermal resistance around a buried horizontal pipe if it is assumed that the surface of the ground is adiabatic. The isothermal ground surface case is approximated for real buried pipes by using the line source method and the mirror image method as for the adiabatic case. In the isothermal case however the image is assumed to be a sink, with equal strength but of opposite sign to the original line source. By using mirror image method together with Kelvin Line Source theory, the temperature difference for horizontal ground exchangers becomes: Q t-t = o 2.n.k [~ICX ) + ICX ) I L 2D J t for adiabatic assumption. And, Q t-t = o 2.n.k I ICX ) - ICX ) 1 [_ r 2D J t XIIIfor isothermal assumption. Where, r = pip© radius D = horizontal pipe depth Thus, soil resistance ar round burried pipes can be more realistic ly determined and the disadvantages of over or undersizing ground heat exchangers can be minimized. XIV

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