Tavandan soğutmalı güneş enerjili absorpsiyonlu soğutma sistemi

Abstract

Bu çalışmada, öncelikle absorbsiyonlu soğutma sistemleri güneş enerjisi ve tavandan soğutma sistemleri hakkında bilgi verildi. Daha sonra bu sistemlerin bir kombinasyonu olan güneş enerjisi destekli absorbsiyonlu soğutma çevriminden yararlanarak bir konutun tavandan soğutma uygulaması yapıldı. Absorbsiyonlu soğutma sisteminin çalışma şartalan seçilerek, her elemanın ısıl yükleri hesaplandı. Bulunan ısıl yüklere göre herbir eleman için ısı değiştiricisi boyutlandırılması yapıldı. Soğutma sisteminin performansını etkileyen parametreler belirlenip, bunları etkileri incelendi ve grafikler halinde gösterildi. Sonuç olarak, tavandan soğutma sisteminin kollektör veriminde ve absorbsiyonlu ısı pompasının veriminde bir yükselmeye sebep olduğu görüldü. Bu da sistemin ekonomik olarak uygulanabileceğini gösterir.

System consists of the pipe that is embedded in the ceiling. The chilled fluid circulate through the embedded pipes. Therefore, the slab is maintained at a colder temperature than the ambient. Hoses manufacture from metal pipes or thermoplastic pipes. Insulation is used to minimize heat gains from the back end parameter of the cooled slab. The head gains from the space establishes its sensible space cooling effect. Head that is gained by the slab, is negative. The required cooling load intensity of space is as following: qs = -Os'/(At*1000) Radiant ceiling panel has to be designed to meet or exceed this magnitude. The reduced peak design load may be shaved off by the cold stored in the radiant slab and partly in enclosing walls. Thus, Qs = C*Qs' where C is the peak load shaving factor. The peak load shaving factor ( C ) is shown Table 1. Table 1. The peak load shaving factor, C The intensity of sensible cooling effect by the ceiling panel depends on the hose, spacing, mean fluid temperature, size of the indoor space altitude and, the indoor design air temperature. The total heat is equal to radiation vu heat plus convection heat. The total heat is extracted by the panel surface of unit area. Qy =qr + qc % = a*Cd*(Tu-Tp) % = ( 1- 2.257*10-5 "% )2.6275 * 2.3 * ( Ta - Tp )l-25 Hose surface which are in contact with the fin are assumed to be isothermal. Edges are insulated. The radiation and convection fin efficiency is defined as following: where tanh (m. W ) n = m.W V(l/Ky+İ/Ka) in = z. >fae. Dd" w= M/2000 The required flow rate in the circuit is: Qs Vs = looo *r* cs *at The velocity of water in the hoses : 4VS vs =. H*Di2 The heat gain of space is n(Td-Ts) 1000 qs = . .x 11 Dd M + -. in( ) a. Di 2. X\y Dj viii The convective heat transfer coefficient between the fluid and the hose wall is : as « 910. ( 0.023. ( Ts + 273 ) - 4.714 ). vs°-8 / Di°-2 Absorption refrigeration systems require only a heat source. This heat is provided by solar energy. This is required large solar collector area and the head pump, therefore, initial investment is high. If the radiant ceiling cooling system is used, the collector efficiency and COP of the absorption heat pumps will increase. If a solar absorption heat pump is to be used ; as the heat exchangers in absorption systems have long time constants, cycling reduces the efficiency. There is a third reason for temperature modulation in solar systems since it allows one to energize the generator at match lower temperatures, and increases the solar collector efficiency and the heat pump COP. Flat plate collectors provide the necessary heat for the generator. Evaporator provides the heat extraction through the radiant ceiling panel circuit. Rejected heat from the condenser and the absorber heat the domestic water. A storage tank may be used. Generally the temperature drop between the heat source and the generator is around IOC. The exit temperature from the solar collectors may be about 80 C. Therefore, the collector efficiency increase. A increase in collector efficiency cause to the required collector area, thus reduce the system cost. In space cooling applications, presence of large quantities of ammonia in building may violate fire and safety codes. Water amonia systems require high solar collector water outlet temperatures. A water - Lithium bromide system is simpler than the water - amonia systemThe lithium bromide absorption cycle provides cooling at a relatively high COP with the moderate temperatures supplied by flat plate solar collectors. Solar energy is collected by the collector loop and stored in the thermal energy storage tank. When the storage temperature is high enough to operate the absorption generator, the absorption generator, the absorption cycle can be energized upon demand is require but solar temperatures are not adquate, heat can be supplied by an auxiliary boiler. Due to low regenerating temperatures required, silica - gel - water absorption systems represents another alternative for radiant ceiling ceiling cooling systems. ix The features of absorption refrigeration system will be explained below. Absorption refrigeration cycle run on low level energy sources such as solar energy and in fact, this is currently their basic advantage. The absorption refrigeration unit is usually divided into a primary cycle and the secondary cycle. The primary cycle consist of the generator, condenser, the evaporator, the expansion valve and the absorber. This is refrigeration cycle. The secondary cycle is compressed of the generator, the absorber, an expansion valve and a pump. This is the cycle, which carries the strong binary solution from the absorber to the generator and returns the weakened solution from the generator to the absorber. The pump in this cycle maintaince the desired circulation flow rate and the expansion valve reduces the pressure of the flow from that of the generator to that of the absorber. This cycle has two pressure : the high pressure which is maintained in the generator and the condenser, and the low pressure which is maintained in the evaporator and the absorber. Conservation laws are applied to the individual components and assuming steady - state conditions and negligible heat losses the following is the conservation of energy equation of the system. Qe + Qg = Qa + Qc where Qe, Q«, Qa, Qe are the heat of evoparator, the heat of generator, the heat of absorber and the heat of cindenser, respectively. Qe is the thermal power absorbed from the surrounding by the refrigerant. All the heat flow rates are computed from the known mass flow rates and the increase of the enthalpy of the flow. The coefficient of performance of the system can be defined in two ways : the ratio of the heat removed from the surrounding the cooling effect, to the solar radiation energy incident on the collectors: or the ratio of the heat absorbed from the surrounding to the heat supplied to the generator. Thus Qe COP = where COP is the coefficient of performance of the system. Calculations with variable parameters led to evaluate COP variation with paramaters. COP shows am increase as the evoparator temperature increases. When evopareator temperature increased so did the boiler dimensions decreased. COP decreased with absorber temperature. On the other hand, the temperature of the generator directly effects the COP of the system and the flat plate solar collector efficiency, when other design conditions are fixed. The required generator temperature depends on the temperature of the chilled fluid. In a water - Lithium bromide system, producing a 5 °C chilled water temperaure instead of 7.5 °C would require an increase in the generator thus heat source temperature of about 5 °C. This effects the radiant ceiling cooling. This system which requires moderate chilled fluid temperatures allow to reduce the energizing temperature. Therefore, the solar collector efficiency will be higher.