THE INSTITUTE OF REFRIGERATION

Transcription

1 Copyright 2015 The Institute of Refrigeration No publication or reprinting without authority THE INSTITUTE OF REFRIGERATION Energy systems for green buildings and solar panel heat pumps by R. Z. Wang, winner of the J&E Hall Gold Medal, and X. Q. Zhai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, (Session ) presented before the Institute of Refrigeration at Climate Center, The Sustainable Building Center, Harrison Way, Spa Park, Leamington Spa, CV31 3HH On Thursday 5 th February 2015 At 2pm This paper will be given as part of a SIRACH event. Places are limited, please contact [email protected] if you wish to attend the presentation Summary In order to achieve reduced energy consumption in green buildings, it is necessary to reduce reliance on fossil fuels by making best use of renewable energy, such as solar energy, wind energy and geothermic energy. In addition efficient technologies including heat pumps and combined cooling, heating and power systems are also effective solutions for green buildings. In this paper, the above-mentioned technologies are introduced mainly based upon the research experience in Shanghai Jiao Tong University. The main performance characteristics of these technologies are reviewed. Finally, the application and performance of several energy systems will be presented based upon the Green Energy Laboratory in Shanghai Jiao Tong University. Proc. Inst. R

2 1. Introduction The concept of a green building has generated extensive interest among building and energy researches all over the world. Green buildings are examples of applied ecology, where designers understand the constitution, organization, and structure of ecosystems, and the impact of architecture is considered from an environmental perspective. By utilizing the concepts, methods, and language of ecology designers can create architecture that intentionally engages the natural system of a site [1]. As for energy consumption of green buildings, it is highly recommended that fossil fuel dependency be reduced by making use of renewable energy, such as solar energy, wind energy, geothermic energy and also heat pumps. New features and trends in energy systems have emerged in recent years. Solar heating or solar hot water supply are becoming common and there is already a huge market. The use of solar heating for winter and solar cooling for summer is a very interesting and topical research area. Various solar cooling systems (absorption, adsorption, desiccant cooling or dehumidification and even solar PV powered vapor compression heat pumps) have been introduced to the market after several years experience of demonstration projects. Heat pumps (air source, water source, ground source and even solar source) have been used efficiently in energy saving buildings, both in residential and commercial applications. Independent temperature and humidity control has been widely adopted to achieve % energy saving, in which a vapor compression system could thus be operated with high evaporation temperature (10-15 C) as the latent heat is treated by a dehumidification process. This concept could even be used for solar cooling at high evaporation temperatures if an independent dehumidification process is arranged. Distributed energy systems (combined cooling, heating and power system, CCHPs), have been recommended for commercial uses in hotels and hospitals if natural gas is available; such systems have been considered as safe and highly efficient because the waste heat from the gas engine can be used for heating or cooling. There are many other examples of high efficient energy systems for green buildings. New designs for HVAC terminals have been introduced to the market. Nowadays a common term in the area of efficient renewable energy usage and also CCHPs is Smart Grids. With a Smart Grid a green building could generate electricity by the use of solar PVs, CCHPs and wind energy etc., forming a micro power plant. If the amount of electricity generated is higher than the required amount, it can be sent to the electric grid network. If it is lower than the requirement, electricity from the grid could be used via a smart meter. In this paper, some widely used energy systems in green buildings are introduced. The application and performance of several energy systems are presented based upon the newly built Green Energy Laboratory (GEL) in Shanghai Jiao Tong University (SJTU). 2. Solar energy systems for green buildings 2.1. Solar heating Solar thermal utilization should be based on the integration of solar collectors into buildings. The facades of buildings can be important solar collectors and therefore become multifunctional. In addition, solar collectors can be used to enhance the appearance of the facade when considering their aesthetic compatibility. Currently, installations of collectors on south tilted roofs, south walls, balconies or awnings of buildings are the feasible approaches for the integration of solar collectors into buildings. Proc. Inst. R

3 Since 1980, solar water collectors have undergone a rapid development with an annual average growth rate of 30 %. By the end of 2013, over 310 million square meter solar collectors have been put into use in mainland China, which probably accounted for 70% of the global solar collector area. Solar collectors are spreading from rural to urban areas. Nowadays, the popularity of solar water collectors in cities attains 25 %. Solar energy, therefore, has an important role to play in building energy systems regarding the development of the solar energy industry in China. Solar collectors can be flat-plate or evacuated tube solar collectors and integrated into building compactly. The Shanghai Green Building (shown as Fig.1(a,b)) project completed in 2005 has shown how solar integrated heating and cooling can be utilized in a building. This project has stimulated a big market for integrated solar systems in China [2,3]. Now such building integrated systems are used in cities and towns and also solar space heating has been accepted in some buildings. SJTU also has experience with balcony integrated solar collectors which are installed in a residential area with 2800 apartments (Fig.1(c)) in Shanghai. (a) Office building (b) Residential building (c) High rise apartment building Fig.1 Solar integrated Shanghai Green Buildings Solar cooling As an environmentally-friendly way of making good use of waste heat and solar energy, sorption cooling systems have drawn considerable attention because they neither use conventional refrigerants as a working substance, nor do they use fossil fuels or electricity as driving heat source. The matured market available sorption cooling systems include silica gelwater adsorption chiller and LiBr-water absorption chiller. Various adsorption refrigeration cycles which use physical sorption working pairs, such as continuous heat recovery cycle, mass recovery cycle, etc, have been investigated at SJTU [4,5]. A 10 kw adsorption chiller was developed and commercialized by SJTU in 2004 (shown as Fig.2(a)). Tests have shown that the solar adsorption cooling system can provide chilled water at a temperature of 15 C from 9:20 to 17:15. The lowest hot water temperature to drive this adsorption chiller is about 55 C. The measured solar collector efficiency, cooling capacity and thermal COP are 0.36, 9.5 kw and 0.44, respectively. The solar COP is about A newly developed 50kW module designed system (shown as Fig.2(b)) came to the market in 2014, which has better performance. There are adsorption chillers (mostly 5-50 kw) sold per year worldwide, which shows the good prospects of solar cooling. Fig.2(c) shows a typical flow chart of adsorption cooling powered by hot water. Proc. Inst. R

4 Figure (a) 10kW Figure (b) 50 kw Figure (c) System Schematics Fig.2 Silica gel-water adsorption chillers. LiBr- H 2O absorption chillers are another option for solar cooling. Currently both large scale and small scale systems are available, with single effect and double effect. Absorption chillers are more reasonable for high capacity (100 kw or more). However, the match between solar collector and absorption chiller is crucial. A single effect LiBr-H 2O absorption chiller supplied by evacuated tube solar collector arrays of in total 82 m 2 has been designed and tested in SJTU. The nominal cooling capacity of the chiller is about 18kW. The chiller developed seems to be feasable, though its thermal COP is about 0.4, which still needs to be improved. The recent focuses of SJTU for solar cooling are solar absorption cooling suited with medium temperature solar collector (80~150 C), a variable effect (1.n effect) LiBr-water absorption cycle has been invented by SJTU, the real machine could have cooling COP from 0.6 to 1.2 according to heating source temperatures ( C). Proc. Inst. R

5 The match of solar collector with sorption system is important. Silica gel-water adsorption chillers can be matched with normal solar water heating units (55-90 C), LiBr- H 2O single effect absorption chillers can match with evacuated tube solar collectors (>80 C), whilst LiBr- H 2O double effect absorption chillers need medium temperature solar systems(>140 C). The newly developed variable effect absorption chiller could meet more high efficiency cooling for medium temperature solar collector (80~140 C) Solar desiccant cooling Desiccant dehumidification cooling systems driven by solar energy are another option for building energy systems, as space heating and cooling should be coupled with space relative humidity control. Both solid-desiccant and liquid-desiccant could be used for handling latent load or even total building space load. SJTU has used rotary wheel desiccant dehumidification for space cooling. The dehumidification system could be powered by solar heating, and the sensible cooling could be powered by a high evaporation temperature (~15 C) chiller, thus the electric COP could be increased by 30% [6]. By developing a new compound desiccant wheel, the generation temperature could be reduced and the dehumidification capacity could be increased. A novel two-stage rotary desiccant cooling system has also been realized by SJTU. Experiment results show that this set-up system needs lower regeneration temperature and has a higher thermal coefficient of performance COP close to 1[7,8]. 3. Heat pump systems for green buildings 3.1. Air source heat pump Because of their high efficiency in energy use, heat pumps have been widely used in HVAC related applications. The research group in SJTU has developed an air source heat pump (ASHP) water heater (Fig.3(a)), which has been demonstrated in residential apartment buildings (as is shown in Fig.3(b)). Demonstration tests have shown that an air source heat pump water heater is better than a solar water heater in high rise residential buildings (102 apartments) due to limited space and temporal availability. Figure (a) water heater Figure (b) demonstration building (102units). Proc. Inst. R

6 Figure (c) residential energy center for heating, cooling and hot water supplies. Fig.3 Air source heat pump (ASHP) heating and its demonstration in Shanghai residential apartments. China has already developed an air source heat pump water heater market with annual sales of 8 billion RMB Yuan per year, in which R22, R134a and R410A are used as refrigerants. A hot water tank usually has litres capacity for residential family use. The heat pump water heater has a heating COP about 3~4 to supply C hot water. In Southern China, where Shanghai is, air source heat pump water heaters are more suitable for cities or towns especially with high rise residential buildings. A newly developed technology of ASHP is it s use as a residential energy center capable of comfort heating, cooling and also hot water supply (schematically shown as Fig.3(c)), in which small temperature difference terminals could be used, 35 C hot water or 10 C chilled water could be used for winter heating and summer cooling respectively. A total energy saving of 20% could be easily reached. For a field test, a commercial ASHP system was installed in the green energy lab. The total airconditioned area is 292m 2. The cooling capacity of the ASHP unit is 45.8kW, with a nominal power of 12.2kW. The whole ASHP system consists of an air-cooled heat pump unit, water pump, water pipes and indoor terminals. Fig.4 is the schematic diagram of the experimental system. In summer, two different indoor terminals including normal fan coil unit (NFCU) and small temperature difference fan coil unit (STDFCU) are combined with the outdoor unit, respectively. In winter, the floor heating coils can also be used besides the aforementioned two terminals. According to the experimental results, ASHP+STDFCU has a 13%-24% higher COP than ASHP+NFCU under similar weather conditions. Proc. Inst. R

7 Figure (a) schematic diagram of the system Figure (b) photo of the ASHP Figure (c) test room Fig. 4 The experimental system for ASHP Ground source heat pump Ground source or geothermal heat pumps are a highly efficient, renewable energy technology for space heating and cooling. This technology relies on the fact that, at depth, the earth has a relatively constant temperature, warmer than the air in winter and colder than the air in summer. Ground source heat pumps are gaining increasing interest because of their potential to reduce primary energy consumption and thus reduce emissions of greenhouse gases [9]. Proc. Inst. R

8 A GSHP system was developed for Minhang archives building of Shanghai. Based upon the 4 year long-term monitoring survey, the indoor environment met the Archives Design Code issued by China national archives. The all year round experimental data showed that the average COP of the heat pump in summer was 4.7, correspondingly, 4.6 in winter and 3.9 in transition seasons. Compared with an air source heat pump system which is widely used in archive buildings in Shanghai, the operating cost of the GSHP system was reduced by 55.8% and the payback time would be two years [10,11]. Our research also showed that small GSHP (capacity about 20-50kW) is not as effective due to the reduced system COP, which might be good for winter heating in a severely cold region River water source heat pump River water source heat pump (RWSHP) is another renewable technology which is similar to the GSHP. Shanghai is located in the lower reaches of the Yangtze River where surface water resource is abundant. The temperature of river water in this district varies from 7 C to 30 C throughout the year, which is generally higher than the ambient temperature in winter and lower in summer. Consequently, compared with the widely used air-source heat pumps in Shanghai, the RWSHP system is more efficient. According to our experiments in GEL, the average COP exceeded 5.0 on a typical summer day. But such systems may have problems for winter heating, low temperature thermal fluid might be needed for reliable efficient heat extraction from water sources Solar panel heat pump Solar panel heat pump systems can be classified into three categories: namely, solar sorption heat pump, solar-related systems (eg. ejection) and solar-mechanical systems. The former two systems are based upon solar thermal utilization and the latter utilizes a solar-powered prime mover to drive a conventional vapour compression system. The solar-powered prime mover can be either a Rankine engine or an electric motor based on solar photovoltaic principle. Fig. 5 shows the solar panel heat pump produced by Midea Air Conditioning Company [12]. Such a system could be either directly DC powered or AC powered with a DC/AC inverter. Micro smart grid could be a good solution for the photovoltaic solar panel heat pump. There are already commercial buildings using solar PV powered centrifugal chiller demonstrations in China. The system is an all DC system with high efficiency and is smart grid friendly. Figure (a) system product. Proc. Inst. R

9 Solar PV Inverter AC Controll er Controller Inverter Battery (b) SJTU test system. (c) SJTU test results Fig.5 Solar panel heat pump. An experimental set-up of such technology was also installed in SJTU. The system consists of PV panels, a controller, an inverter, a lead-acid battery bank and a commercial variable frequency split type room air conditioner. The photovoltaic array was composed of 16 photovoltaic panels. In this system, 48 V was chosen as the rated voltage in the direct current circuit, hence, 4 photovoltaic panels were connected in series as a group, then 4 groups of photovoltaic panels were connected in parallel. All the PV panels were positioned in the same azimuth angle with the construction, at an inclined angle of 34 which is the optimum tilt angle to collect solar energy for electricity generation in the Shanghai area. A variable frequency air conditioner was used as alternative current load, which has several advantages compared with the conventional air conditioner, such as soft-start to minimize the sudden impact on electrical energy source; quick response to room temperature; and energy saving due to the ability to operate at a lower frequency after reaching a set temperature, which further avoids frequent starting and stoping of the machine. The rated input power of the air conditioner is 1.4kW. According to the experimental data, the system can work steadily to meet the cooling/heating load at different peak times. The system solar COP is between 0.3 to The solar fraction of the present system was over 80% in normal summer daytime as well as winter daytime. The grid connected solar panel heat pump might be a good solution to reduce the grid fluctuation as well as peak load generation capacity. 4. Combined cooling, heating and power system In a green building the combined cooling, heating and power (CCHP) system has been generally recognized as a solution to meet and solve energy-related problems where gas or other fossil fuels are available. Electricity is usually generated via a gas turbine or a gas engine (both internal or Stirling types). It has been proven that gas engines are very effective for small scale systems (200 kw or lower). The waste heat from the engine could be used for heating and hot water supply. The waste heat could even be used to drive a sorption chiller for cooling or dehumidification. SJTU has developed a moveable micro-cchp system using a small-scale generator set driven by a gas engine and a small-scale adsorption chiller, which has a rated electricity power of 16 kw, a rated cooling power of 9 kw and a rated heating capacity of 28 kw [13,14]. The above concept integrated system has been demonstrated in a hotel in which 150 kw electricity power is generated with a gas engine and 700 kw absorption cooling supply has Proc. Inst. R

10 been reached with the power from the waste heat of the engine and added gas combustion for the high pressure generator of a LiBr- H 2O double effect system. The most critical thing for a CCHP is whether the electricity generated could be accepted by the state grid network. A green building could be powered by natural gas, in which a CCHP is used, thereby electricity can be used directly, and so is the waste heat for heating/cooling/hot water supply. A conservative design for a CCHP is that the gas engine may have a maximum electricity supply which meets the demand of the minimum requirement of the building. The electricity still needed could then be provided by the grid. A smart grid solution is favorable in order to couple the CCHP properly with the network. If this system could be integrated with solar heating or cooling or even solar PV and wind energy and etc., it would be an ideal energy system. 5. Experience in the green energy lab of Shanghai Jiao Tong University The green energy laboratory (GEL) inside SJTU campus, is composed of three elevation floors, with a total construction area of 1500 m 2. As shown in Fig. 6, the first two floors have labs, meeting rooms, staff rooms, student rooms, exhibition atrium, etc. The third floor is designed to be a residential space, and divided into two independent apartments according to typical residence structures in China. This floor is a platform to simulate residential living conditions and to perform tests on energy efficient facilities and building envelopes, also a net zero energy apartment is for demonstration. Figure (a) GEL outlook in SJTU campus Figure (b) energy systems in the GEL Fig.6 GEL with various green energy systems. Proc. Inst. R

11 The whole building space is divided into three kinds of experimental areas for different energy systems. The first one is a main system for the whole space on the 1st floor and 2nd floor. The main system supplies year-round cooling and heating energy for GEL s daily function and also has enough power capacity to meet some special peak energy demands. The second kind contains assistant systems for a lot of individual lab spaces. Various assistant HVAC experiment devices are connected to indoor terminal units in parallel with the main system. On most days, the main system is shut down and lab spaces are served directly by assistant systems for experimental purposes. The last kind consists of two independent systems which are used on the third floor for the two apartments. These two spaces are not connected to the main system but are used for integration system experiments about home energy management systems (HEMS) for zero energy apartment and micro-smart gird technology [15]. The main system is a combination of river water source heat pump (RWSHP) and micro CCHP system. Because the GEL is close to a river, a water source heat pump has been chosen as the core of the main system in order to meet the requirements of cooling and heating loads of GEL. The RWSHP has a cooling capacity of 85.7 kw and a heating capacity of 92 kw. Compared to conventional split air-condition systems (SAC) which are commonly used on campus, the energy-saving ratio and CO 2 emission reduction of RWSHP is 46.53% and 21.41%, respectively. In reality not so much energy input is needed by GEL in normal operation time because the building is always in the partial load condition. So, one micro-cchp with a small gas engine (28kW) and absorption chiller (20kW) is launched alternatively for the ordinary low energy demand of GEL, while the main system is shut down in order to save energy. The micro CCHP is also integrated with 6kW solar PV and 5kW wind energy. Besides the main system, there are several sub systems for conducting experiments in different labs, such as solar adsorption air-conditioning, solar desiccant wheel cooling system, slurry cooling energy storage, ASHP heating and cooling with small temperature difference terminals (fan coil units and radiation cooling and heating), ASHP system with independent temperature and humidity control. Most of them focus on how to integrate low grade energy utilization and emerging refrigeration technology into small commercial buildings and especially residential buildings. Independent energy systems without connection to the main system are utilized for two demonstration apartments (A and B) on the third floor. The indoor areas of these are 58 m 2 and 93 m 2 respectively. The structure of indoor space simulates a typical layout of a Chinese household. Apartment A is a demonstration for a smart house. A VRV air conditioning system with the rated cooling capacity of 10 kw was installed. The power consumption of all the systems including air conditioning, lighting and domestic appliances can be continuously monitored. The air conditioning system can therefore operate with an optimized mode according to the weather condition and the load inside the house. It is concluded that the energy saving rate of the air conditioning system attains 15%. In one full year continuous operation for apartment A consumed 3500kWh. As for Apartment B, it is a net zero energy apartment (NZEA) with a high performance building envelope and lead-edge HVAC system. A 8 kw air-cooled hybrid heat pump which uses solar thermal energy to assist traditional electricity driven compressor cooling device has been developed for this NZEA. The main parts of this device are small scale air cooled LiBr- H 2O Proc. Inst. R

12 absorption chillier and CO 2 heat pump. Application of solar thermal driven LiBr-H 2O absorption chiller can dramatically decrease the outlet temperature of working fluid when CO 2 leaves the gas cooler so that the hybrid heat pump can get a higher COP. The net zero energy apartment B shows a 3 kw solar panel PV would be enough for a local Shanghai residential family uses. Fig. 7 Small compact air source heat pump system (3hp ) installed in demonstration apartment B (93m 2 ) by using 4 small temperature difference fan coil units and a hot water tank Recently, another small compact air source heat pump system (3hp compressor) with the rated cooling capacity of 7.4 kw, capable of heating, cooling and hot water supply was also installed in Apartment B, in which 4 small temperature difference fan coil units were used (2 for bed rooms, two for living rooms). As is shown in Fig. 7. Based upon the experimental results, the indoor thermal environment is comfortable, especially in winter, the indoor temperature is always uniform and kept at C or so, the winter daily electric consumption is about 38 kwh. This technology is considered as a potentially effective approach for the residential buildings in the hot summer and cold winter in some areas in China. 6. Conclusions 1. Shanghai is characteristic of subtropical monsoonal climate with the mean annual temperature of 17.6 C, and receives annual total radiation above 4470 MJ/m 2 with approximately 2000 h of sunshine. Based on the experimental data and our previous experience, it is concluded that solar fraction for the solar cooling and heating is about 70% and 50%, respectively. 2. Shanghai is located in the lower reaches of the Yangtze River where surface water resource is abundant. Therefore, RWSHP is considered as a rational technical scheme on condition that there is a river besides a building. 3. The air source heat pump together with small temperature difference fan coil units are energy-saving and comfortable. They are considered as a potentially effective approach for the residential buildings in the hot summer and cold winter in some areas in China. 4. The grid connected solar panel heat pump might be a good solution to reduce the grid fluctuation as well as peak load generation capacity. 5. Owing to its high primary energy ratio, CCHP is a competitive technology in green buildings in the fact of stable natural gas supply. 6. It is feasible to provide an integrated and flexible energy system for green buildings based on different indoor space types and special application demands. The passive sustainable design can be thought as a good base for green buildings. The renewable and efficient energy system can further perfect the whole performance of the building. In the GEL of SJTU, fourteen up-to-date technologies are used to highlight the performance of the whole building. The ratio of renewable energy to whole building input energy attains 75%. Proc. Inst. R

13 Compared to the performance of conventional building in shanghai, the energy-saving rate of GEL is about 55.4 %. Acknowledgements This work was supported by The Italian Ministry for the Environment, Land and Sea and Marie Curie Actions - International Research Staff Exchange Scheme (IRSES) under the contract No The author also thanks J&E Hall for awarding of the prize. References [1] Victor, Olgyay, Julee, Herdt, The application of ecosystems services criteria for green building assessment, Sol Energy,2004, 77(4): [2] Zhai, X.Q., Dai, Y.J., Wang, R.Z., Experimental investigation on air heating and natural ventilation of a solar air collector, Energ. Build., 2005, 37(4): [3] Wang, R.Z., Zhai, X.Q., Development of solar thermal technologies in China, Energy, 2010, 35(11): [4] Wang, R.Z., Adsorption refrigeration research in Shanghai Jiao Tong University, Renew Sustain Energy Rev, 2001,5(1):1-37. [5] Wang, D.C., Wu, J.Y., Xia, Z.Z., Study of a novel silica gel water adsorption chiller. Part II. Experimental study, Int J Refrig, 2005,28(7): [6] Jia, C.X., Dai, Y.J., Wu, J.Y., 2006, Analysis on a hybrid desiccant air-conditioning system, Appl Thermal Eng, 2006, 26(17-18): [7] Ge, T.S., Li, Y., Wang, R.Z., Experimental study on a two-stage rotary desiccant cooling system, Int J Refrig, 2009, 32(3): [8] Jia, C.X., Dai, Y.J., Wu, J.Y., Use of compound desiccant to develop high performance desiccant cooling system, Int J Refrig, 2007, 30(2): [9] Abdeen, Mustafa Omer,Ground-source heat pumps systems and applications, Renew Sustain Energy Rev, 2008,12(2): [10] Yu, X., Wang, R.Z., Zhai, X.Q., Year round experimental study on a constant temperature and humidity air-conditioning system driven by ground source heat pump, Energy, 2011, 36(2): [11] Zhai, X.Q., Yang, Y., Experience on the application of a ground source heat pump system in an archives building, Energ. Build., 2011,43(11): [12] Lv, G.Z., Li, Y., Solar cooling by solar panels, Sol Energy (in Chinese), 2011,3: [13] Kong, X.Q., Wang, R.Z., Wu, J.Y., Experimental investigation of a micro-combined cooling, heating and power system driven by a gas engine, Int J Refrig, 2005,28(7): [14] Huangfu,Y., Wu,J.Y., Wang,R.Z. Kong, X.Q., Evaluation and analysis of novel micro-scale combined cooling, heating and power (MCCHP) system, Energ Convers Manage, 2007, 48(5): [15] Wang, R.Z., Zhai, X.Q., Energy Systems in Green Buildings (in Chinese), Shanghai Jiao Tong University Press, (2013). Proc. Inst. R