CASE STUDY REGARDING ENERGY EFFICIENCY OF A VENTILATION SYSTEM WITH RECUPERATIVE HEAT RECOVERY

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1 JOURNAL OF APPLIED ENGINEERING SCIENCES Article Number: 131_VOL.3(16), issue 2_2013, pp ISSN ISSN-L (Print) / e-issn CASE STUDY REGARDING ENERGY EFFICIENCY OF A VENTILATION SYSTEM WITH RECUPERATIVE HEAT RECOVERY DOMNIŢA Florin*, HOŢUPAN Anca, ABRUDAN Ancuţa, KAPALO Peter** Technical University of Cluj-Napoca, * florin.domnita@insta.utcluj.ro (corresponding adress) **Technical University of Košice, Slovakia A B S T R A C T This paper refers to a method of heat recovery that uses a plate-type heat exchanger together with an additional air-to-air heat pump. These devices are part of a controlled central domestic ventilation system for single family homes with m 2 of living space. The amounts of energy saving rises up to 60% compared with the energy consumption of a central domestic ventilation system without heat recover. That means that heating demands will be adequately met by the air-to-air heat exchanger/heat pump combination. This system provides most favorable costs for energy consumption for heating or cooling a house and even the investment costs are high, the amortization period is short enough. Received: 23 July 2013 Accepted: September 13, 2013 Revised: November 01, 2013 Available online: December 15, 2013 Keywords: ventilation, heat, pump, recovery, air-to-air, exchanger, system, plate-type. INTRODUCTION In recent years it is found more frequent use in construction of tight closure elements (doors, windows) that do not allow air to circulate from outside to inside the building and vice versa. Also, standards of EU countries and especially energy-saving rules that come into force in 2012 provide a further reduction of 20 to 25% in heating demands. In such case, buildings will not be able to function without some system of mechanical ventilation and the transmission heating and ventilation will be provided exclusively by mechanical ventilation systems. This paper objective is to propose a way of reducing energy consumption for heating (cooling), in small mechanical ventilated domestic buildings, that uses two combined methods in order to obtain a better heat recovery: plate-type heat exchanger and air-to-air heat pump. Starting with the study of german norms and based on the selected bibliography, the paper presents a double heat recovery system adapted for a residential home, together with the methodology for calculating the energy efficiency of the system. MATERIALS AND METHODS Mechanical installations for supplying air to and extracting air from domestic buildings will become standard and methods of heat recovery, the norm [1]. That will mean first and foremost, recuperative equipment (exchanger plates) and heat pumps to recover the energy still in the extracted air. Just as in the conventional heat recovery equipment, the cooled exhaust air has enough heat enthalpy which can be further used by a heat pump thereby making contribution to energy savings. Energy will be extracted from the extract air with an air-to-air heat pump into a heat exchanger (evaporator) and raised to a higher temperature using an electric compressor. At this high temperature, all the heat will be emitted through a heat exchanger (condenser) to the supply air which has been pre-warmed by the heat recovery equipment [2]. The equipment is a combined supply and extraction air device equipped with an additional heat pump component in which evaporator and condenser, together with the compressor and the necessary components form one module unit [3]. The outer casing can be easily removed allowing the heat pump component to be inserted laterally into the basic equipment. Due to the symmetrical 27

2 DOMNIŢA F., HOŢUPAN A., ABRUDAN A., KAPALO P.: CASE STUDY REGARDING ENERGY EFFICIENCY OF A VENTILATION SYSTEM WITH RECUPERATIVE HEAT RECOVERY lay-out of the exchanger plate, when switched over, the module can be used as a cooling device in summer. Fig. 1. Controlled central domestic ventilation systems with recuperative heat recovery by using plate-type heat exchanger and air-to-air heat pump In Figure 1 it shows the position of the heat exchanger plates and heat pump elements. The ventilator is positioned on the pressure side. If it were mounted on the low pressure side, a costly diffuser would be necessary. The exhaust fans, respectively the supply fans are mounted after the evaporator, respectively after the condenser [3]. The refrigerant circulation is identical to the configuration and type of refrigerant (R 134 A) which up to now has provided to be reliable in practice. That corresponds to a surface temperature of the chiller of approximately 7ºC. A reversal of the refrigerant circulation eliminates the need to move the module round to the other side of the equipment which would incur extra time and costs. In calculus were taken the following conditions as basis: Volume flow D of the basis equipment: 3 m D = 250 h = 300 h Power consumption of fans: Q F = 95W in the working point Indoor air parameters from house: - temperature: t i = 22ºC; - relative humidity: φ i = 60%; - enthalpy: Intake air parameters from outside: - temperatuture: t e = 5ºC; - relative humidity: φ e = 30%; - enthalpy: h i = 47, 3 h e = 9,1 28

3 JOURNAL OF APPLIED ENGINEERING SCIENCES Article Number: 131_VOL.3(16), issue 2_2013, pp ISSN ISSN-L (Print) / e-issn Temperature difference between inside and outside from house: Δt = 17 K Return temperature coefficient: Φ R = 0,64 (exchanger plate data) RESULTS It results the following calculations: Supply air temperature after the heat exchanger(hot side): o th = te + ΦR Δt = 15,9 C From h-x diagram it results: Supply air relative humidity after the heat exchanger (hot side): φ h = 12% Supply air enthalpy after the heat exchanger (hot side): h h = 20 Enthalpy difference between supply air and intake air: Δhsup ply = hh he = 10,9 Next, the heating power/cooling power of the plate type heat exchanger [4, 5] will be: QHE = D Δhsup ply = 908W The air supply temperature increase through the plate-type heat exchanger with [4, 5]: Q HE 908 3,6 ΔθHE = = = 10,9K D 300 Cooling of the extract air in the heat exchanger for the same airflow rate occurs with the same enthalpy difference: Δhextracted = Δhsup ply = 10,9 Exhaust enthalpy according to the plate-type heat exchanger (cool side) will be [5]: h = he Δhextracted = 36,4 From h-x diagram it results: Exhaust air temperature according to the plate exchanger: t = 13ºC; Exhaust air relative humidity according to the plate exchanger: φ = 100%. It is calculated the cooling power of the evaporator in the heat pump Q HP [6], considering that surface temperature of the chiller is around 7ºC, so the air enthalpy near the surface of the chiller is : h c = 22,7 So: QHP = D Δh = D ( h h c ) = 1191W This cooling power is achieved through the optimal, geometrical qualities of the equipment. Where the amount of space is limited, e.g. due to the type of equipment or additional fitments, the cooling decreases accordingly. The air supply temperature increase through the condenser of the heat pump with: 29

4 DOMNIŢA F., HOŢUPAN A., ABRUDAN A., KAPALO P.: CASE STUDY REGARDING ENERGY EFFICIENCY OF A VENTILATION SYSTEM WITH RECUPERATIVE HEAT RECOVERY Δ θ Q HP HP = = 14,3K D In the case of suction gas-cooled compressors, almost all energy is converted into heating power. Considering that the compressor power Q at a power coefficient of 4 is around 350W, it can calculate [6]: Heating power of the condenser: Q C = Q HP + Q = 1540W Efficiency indication of the heat pump: Qc ε HP = = 4,4 (hot side) Q Power rating of complete equipment: QC + QHE + Q ε = F = 4,67 Q + 2 QF The air supply temperature increase by the suction gas-cooled compressor with: Q Δ θc = = 4,6K D Also, the air supply temperature increase through the supply air fan with: Q Δ θ F F = = 1,5K D Total temperature increase on supply air side through heat recovery equipment, heat-pump, compressor and fan with [6]: Δ θtot = ΔθHE + ΔθHP + ΔθC + ΔθF = 29,8K Supply air temperature on to house will be: o t = t e + ΔθTOT = ,8 = 34,8 C Total heating power of heat recovery, heat pump at the appropriate conditions (see above) will be: QTOT = QHE + QHP + QF = 2543W CONCLUSIONS Single family homes or flats with m2 of living space will not exceed maximum heating demands of approximately 6000W in the future [7]. Almost half of this demand can be provided by controlled central domestic ventilation system with recuperative heat recovery by using plate-type heat exchanger and air-to-air heat pump. Using this heat recovery system, will result in a balance of minimum +5ºC for indoor air temperature for the whole cold season. That means that heating demands will be adequately met by the heat recovery/heat pump combination (fig. 2). As outdoor temperatures increase, the heat recovery equipment continues to function and the heat pump comes on at intervals. As temperatures fall, the heat pump functions continuously. Additional heating must supply the remaining requirements. The system with recuperative heat recovery by plate-type heat exchanger and air-to-air heat pump will work in parallel with a radiators heating system up to temperatures of 4-6ºC, after that it will be disconnected. Using the system described above in parallel with a radiators heating system, at an outdoor temperature of -12ºC, the supply air temperature passing through the equipment will still be higher than the room temperature, resulting in energy savings of at least 50%. 30

5 JOURNAL OF APPLIED ENGINEERING SCIENCES Article Number: 131_VOL.3(16), issue 2_2013, pp ISSN ISSN-L (Print) / e-issn Additional methods of heat generation with low power (3 5 kw - electric, gas or oil) and low surface temperatures can be use. Fig. 2. Comparison between central domestic ventilation system with recuperative heat recovery with plate-type heat exchanger and air-to-air heat pump and other heating systems In the same Figure 2 it can be seen how much energy can be saved by using a heat recovery system with a single air-to-air heat-exchanger in parallel with traditional heating system. Another advantage of this system is the possibility of switching the heat pump connections to supply air duct, respectively to exhaust air duct. Thus, the cycle is reversed and the system will be used during the warm season to save an equivalent of cooling energy, which will be recovered in two steps with the aid of plate-type heat exchanger and air-to air heat pump in order to additional cooling the air. Also, it can be calculated how many days in the year this additional heating or cooling must be turned on, or the percentage of heating or cooling demands than can be met annually with the combined heat recovery and heat pump equipment. As can be seen from the graph, the energy savings are impressive. In conclusion, this system will be surely and frequently used especially for houses and apartments as a way of adapting classical heating to future requirements of energy savings, because it can reduce energy consumption required for heating and cooling with 40-60%, justifying the relatively high investment costs. In the past, the design and installation of heat pumps presented numerous other problems with regard to installation, hydraulic connection, buffer size and passage of water or the air on the primary and secondary circuit [8]. The on-off cycles caused by insufficient transfer of energy were also damaging to the compressor [8]. With the air-to-air heat pump described above, these problems do not arise, as air is provided supply and extraction fans are in operation a large buffer and therefore constant transition is ensured. ACKNOWLEDGMENTS This article was created by implementation of APVV project entitled: Energy Efficiency of the Ventilation Systems. Project number: SK-RO_

6 DOMNIŢA F., HOŢUPAN A., ABRUDAN A., KAPALO P.: CASE STUDY REGARDING ENERGY EFFICIENCY OF A VENTILATION SYSTEM WITH RECUPERATIVE HEAT RECOVERY REFERENCES 1. *** DIN 1976 (2008), Domestic Ventilation. 2. *** DIN 4108 (2009), Heat Conservation in Buildings. 3. DOMNITA, F. (1997), Heat saving system in mechanical ventilated rooms by using an air-to-air heat exchanger and a heat pump - Modern Science and Energy Conference Proceedings, Cluj-Napoca, XVI th Edition, pp HANS-LORENTZ, F. (2009), The air-to-air heat pump with recuperative heat recovery in controlled domestic ventilation systems - Villingen - Schwenningen Conference Proceedings, Karlsruhe, pp DUŢĂ G., COLDA IOLANDA, STOIENESCU P., ENACHE D., ZGAVAROGEA M., HERA D., DUŢĂ ANCA (2002), Manualul de instalaţii; Instalaţii de ventilare şi climatizare (Building Services Handbook; Ventilation and air conditioning systems) Ed. Artecno Bucureşti. 6. CLARKE, J. A. (2001), Energy Simulation in Building Design - Butterworth-Heinemann, Oxford, UK, 2nd edition, pp POPOVICI T., DOMNITA, F., HOŢUPAN ANCA (2011), Instalaţii de ventilare si condiţionare (Ventilation and air conditioning), Volume II, Cluj-Napoca, Editura U.T.PRESS, Chapter *** ASHRAE Handbook Fundamentals (2009), Residential Cooling and Heating Load Calculation - ASHRAE, Atlanta, Chapter