Air Conditioning Systems and Energy Consumption

Transcription

1 Evaluation of energy demand of air-conditioning systems based on probability theory LÁSZLÓ KAJTÁR, MIKLÓS KASSAI Department of Building Service Engineering and Process Engineering Budapest University of Technology and Economics Bertalan Lajos street 4-6. Budapest, H- HUNGARY Abstract: - In accordance with EPBD, energy certification of buildings has been compulsory in Hungary since Additionally primary energy demand of heating, air-conditioning, ventilation and lighting shall be calculated separately as well as energy performance of buildings. In this article it is given a review of an evaluation method of energy demand of air-conditioning systems based on probability theory. Based on probability theory a new calculation procedure has been developed, in order to determinate primary energy demand of air-conditioning systems. To achieve this, outdoor climatic parameters (temperature, humidity, enthalpy) has been considered as varieties. Using this method, comparison between alternative air-conditioning systems can be made during the design process. Since annually fluctuation of outdoor condition is taken into consideration, unnecessary over-dimensioning can be also avoided. Moreover the most effective system can be selected from primary energy consumption point of view. Key-Words: - Air-conditioning systems, Evaluation of energy demand, Probability theory Introduction rom the perspective of air conditioning technology the key attributes of outdoor air are temperature, humidity and enthalpy. These values randomly vary from minute to minute. The probability theory starts from the bulk of the elementary results of some experiment (e.g.: air temperature measurement). One single real number can be attributed to each elementary event (the measured temperature). The correlation interpreted with this attribution is called the probability variable. We can talk about discrete and continuous distribution probability variable. The air condition values can take any value within a range and may be regarded as a continuous probability variable. A probability variable may have an expected value, dispersion, distribution and density function. A probability variable with a continuous distribution is characterized by the distribution ((x)) and density (f(x)) functions. The probability that the probability value is between a and b : = P ( a< x< b) f ( x) dx b a () The distribution function shows the probability of t probability variable taking a value less than x : ( x) = P( t< x) (2) The distribution functions of outdoor air parameters (temperature, enthalpy) can be applied for determine the actual energy demand of air-conditioning systems. The temperature, humidity and enthalpy of outdoor air can be accounted continuous distribution probability variable that can be converged as Gauss distribution. Calculation of heating demand it is necessary to take account of variation of outdoor air parameters (temperature, humidity and enthalpy) that vary in daily and season period. Making energy calculation of a building, the outdoor air condition values can be regarded as a probability variable that has a momentary, maximum, minimum and average values, distribution function which can be defined according daily, weekly, monthly and annual period. We can make more refinements by define a daytime period (07-9 hours) and a continuous operation (0-24 hours) of airconditioning systems. Two distribution functions are shown in figure., in that a daytime (07-9 hours) period and nightly period (9-07 hours) can be seen. Any point of functions shows the hours and the percentage of the term when temperature or enthalpy of outdoor air was lower than the examined point. It follows that the τ tot term which concern to the 00% distribution is equal to the reference period. By integral calculation the air handling process is taken into account in Mollier enthalpy chart. Integral always can be calculated by the area under the functions during determination of cooling and heating energy consumption. In view of annual energy ISSN: ISBN:

2 consumption operational costs can be calculated by knowledge of power cost and heat cost. 2 Methods An air handling unit contains two main groups of elements: supply and exhaust units. The main parts of an air handling unit: filter, heat recovery unit, preheater, re-heater, cooler, by-pass, adiabatic humidifier, fans. In this article determination of cooling energy consumption in winter time is published. rom the above-mentioned elements have the possibilities to build up different air handling units. The main parts of energy consumption are: heat energy, cooling energy, ventilation and pumping power. Calculations of these parameters are really complicated by variable efficiency operation of air handling units due to fluctuation of outdoor condition. Determining of energy consumption pumps and fans of AHUs is not complicated whereby they operate by constant efficiency. Heating energy consumption can be calculated by distribution and density functions of outdoor temperature and enthalpy. During the desing process it is important to taking into account the succession of elements in air handling units and accordingly the character of the air handling process in Mollier enthalpy chart. Hereinafter sizing of a representative air handling process in winter operation is published. ig.. The distribution function of outdoor air temperature from October until March (Budapest, average of measured temperatures between ) 2. Air handling unit with heat recovery The applied signs are the followings: : pre-heater : re-heater C : cooler AH : adiabatic humidifier Operation parameters of air handling unit with heat recovery, are shown figure 2. It shows the distribution function of outside air temperature from October until March. The signs of the figure are the followings: (t o ) : distribution function of outdoor air temperature T ; o C : air temperature after the pre-heater T I ; o C : indoor air temperature (equal to the outgoing air temperature - T out - ) T s ; o C : (t o ) : supply air temperature air temperature after the heat recovery which can be calculated by indoor and outdoor air temperature and technological parameters of heat recover unit. ig.2. resh air supplied by air handling unit with heat recovery, pre-heater and re-heater in winter time [] Energy consumption of pre-heater: = ρ c pa t o dt ; (3) ρ; kg/m 3 air density c pa ; kj/kg. C specific heat of air on fixed pressure ; m 3 /h air volume in pre-heater ISSN: ISBN:

3 τ tot term which concern to the 00% distribution is have to be used for air handling units that are in continuous operation (0-24 hours). Distribution that apply to half day is have to be used for air handling units that are intermittent-duty (07-9 hours). Energy consumption of re-heater: t S = ρ c pa V o dt t ; (4) ; m 3 /h air volume in re-heater The areas which shows ig.2. are proportional to energy consumption of the elements of air handling unit (, ) and the energy saving of heat recovery. In a similar way there is possibility to set up a physical and mathematical model of other air handling unit Energy consumption of air handling unit with by-pass Determining of energy consumption of AHU with bypass is a similar process than in the case of fresh air supplied by AHU. The difference between the two cases is necesarry to take account of the effect of the air recirculation during the air handling process. The proportion of air recirculation can be calculated by air volume currents (igure 3. and 4.): rec SZ a = a+ b ig.3. Air handling unit with by-pass, heater and without adiabatic humidifier in winter time In addition igure 3. shows that supply air temperature is not constant under the full season it follows the dercease of the heat loss. On the end of the heating season supply air temperature and outgoing air temperature is equal. Energy consumption of heater: H ( t t ) t I S I τ tot = ρ c pa V ( to) dt+ ; kj/year Air handling unit with by-pass, heater and without adiabatic humidifier The order of the air handling elements is: BP + H The energy consumption can be calculated by the distribution function of outdoor air temperature. (igure 3.). In accordance with the proportion of air recirculation the temperature distribution function after the mixing of fresh air and recirculated air can be determined by temperature of indoor air and outdoor air ( (t o ) ) Air handling unit with by-pass, pre-heater, re-heater and adiabatic humidifier The order of the air handling elements is: + BP + AH + The air temperature after the adiabatic humidifier (t AH ) can be determined by winter air handling process on Molliere enthalpy chart. ig.4. shows the energy consumption of air handling elements. ISSN: ISBN:

4 3. Heat recovery, pre-heater, adiabatic humidifier, reheater 4. Heat recovery, pre-heater, by-pass, adiabatic humidifier, re-heater The last mentioned air handling process is the most complex. Air handling processes are known in Mollier enthalpy chart. Results of the calculations show that there is no need for pre-heater in this model because heat recovery can cover the designed temperature which generates the pre-heater in designed state as well. 4 Conclusions Daytime and nightly energy consumption of the four air handling units have been analyzed by the developed methods from October until March. (ig.5.) ig.4. Air handling unit with by-pass, pre-heater, adiabatic humidifier and re-heater in winter time Energy consumption of pre-heater: = c pl ; m 3 /h : t dt ρ ; kj/year O air volume in pre-heater Daytime energy consumption oktmarc. [MJ] Air handling units Energy consumption of re-heater: t = I ρ cpl tah ; m 3 /h : O dt+ air volume in re-heater ( t t ) S 2 I τ tot The calculation of heating energy consumption can be seen in the above-mentioned air handling units. Energetic analysis can be made in any air handling units with the help of the presented methods and equations. 3 Results We have examed air handling units that contain the following elements, in a comparative energy analysis:. Pre-heater, adiabatic humidifier, re-heater 2. Pre-heater, by-pass, adiabatic humidifier, re-heater ig.5. Daytime energy consumption (7:00-9:00) Total energy consumption of the four air handling units from October untill March shows Table. Based of total energy consumption, it is possible to estimate the energy savings for each AHUs. The reference air handling unit supply only fresh air, without by-pass and heat recovery (. version). The results can be seen in Table 2. Results show, that energy savings are significant in case of using by-pass and heat recovery. Applying both methods rate of energy saving can be 79%. (4. version) Cooling energy consumption can be calculated by developed mathematic methods in a similar way in summer. References: [] Kajtár L Klímatechnikai rendszerek energetikai, gazdasági elemzése valószinűség- ISSN: ISBN:

5 elméleti alapon. Budapest: 7. űtés- és [4] Kajtár L.-Vörös Sz Risk-Based Modelling Légtechnikai konferencia CD kiadvány 2p. of Air-Conditioning System in Hungary. [2] Kajtár L., Kassai M Levegőkezelő Coimbra, Portugália: ROOMVENT 2004, 9th központ energiafelhasználásának elemzése International Conference on Air Distribution in valószínűségelméleti módszerrel. Budapest: Rooms. Book of Abstracts p. CD 6p. Magyar Épületgépészet [5] Kiss R Légtechnikai adatok.!!!!!!! Budapest: [3] Kajtár L., Vörös SZ. 2004: Klimatechnikai rendszerek kockázati elvű méretezése. Budapest: 6. ütés- és légtechnikai konferencia CD kiadvány 5p. Műszaki Kiadó [6] Monostory I Valószínűségelmélet és matematikai statisztika, Budapest: Műegyetemi Kiadó Table. Total energy consumption (00-24 hours).+ah+ 2.+BP+AH+ 3.HR++AH+ 4.HR+()+BP+AH+ Heat energy [MJ] Pump + an en.[mj] Total [MJ] Table 2. Rates of energy savings in [MJ] and [%].+AH+ 2.+BP+AH+ 3.HR++AH+ 4.HR+()+BP+AH+ Total [MJ] Total [%] ISSN: ISBN: