Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation

Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation

G Model ARTICLE IN PRESS ENB-5002; No. of Pages 9 Energy and Buildings xxx (2014) xxx–xxx Contents lists available at ScienceDirect Energy and Bu...

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G Model

ARTICLE IN PRESS

ENB-5002; No. of Pages 9

Energy and Buildings xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation A. Real, V. García, L. Domenech, J. Renau, N. Montés, F. Sánchez ∗ Department of Building Engineering and Industrial Production, University CEU Cardenal Herrera, Valencia, Spain

a r t i c l e

i n f o

Article history: Available online xxx Keywords: PCM Heat pump Thermal energy storage Solar Decathlon Europe HVAC system

a b s t r a c t This paper exposes a HVAC system installed and tested during the Solar Decathlon Europe 2012 competition at the SMLsystem prototype. This system is focussed to improve the performance of a heat pump based HVAC system with two thermal storage tanks using phase change materials (PCM). A cold tank is used to take advantage of the low outside temperatures at night to cool the PCM with a high COP and it is used later to cool the building when the outside temperature rises. The second tank is operated as an alternative hot reservoir which provides the system the flexibility to dissipate the heat to the tank at a constant temperature granting never to reduce the COP from a minimum value. Different functioning strategies to evaluate the efficient use of the HVAC system for cold accumulation and heat dissipation are shown and implemented. © 2014 Elsevier B.V. All rights reserved.

1. Introduction This work presents results of the participation of the Department of Building Engineering and Industrial Production of the University CEU Cardenal Herrera in the Solar Decathlon Europe (SDE) 2012 with the project SMLsystem (see Fig. 1). The approach of the competition, affecting a multitude of innovation facets in the development of efficient houses, yield a multidisciplinary team in which researchers from different disciplines could join forces for a common goal in relation to the efficient use of energy resources in Buildings. A common objective of the participating teams was to design and build prototype houses that reduce energy consumption and obtain all the necessary energy from the sun. During the final phase of the competition, teams assembled their houses in Madrid, and competed during September 2012 in ten different contests regarding energy savings and sustainable construction. In that sense, the SDE rules are intended to achieve the society aims and the organization purposes to encourage a fair and stimulating competition among participant teams including the scoring distribution and the contests evaluation criteria and procedures. During the contest week, the houses were continuously monitored and

∗ Corresponding author. Tel.: +34 961369000; fax: +34 961300977. E-mail addresses: [email protected] (A. Real), [email protected], [email protected] (F. Sánchez).

specific measurements were also done. SDE teams and general public had access to all the information related to the houses. One of the main goals was to demonstrate that high performance solar homes could be comfortable and affordable. In terms of comfort conditions, the prototypes had to assess the capacity to provide interior comfort through the control of temperature, humidity, acoustic, lighting and the quality of the interior air. The interior temperature was constantly measured and all obtainable points were received at the conclusion of each scored period depending on the house behaviour. Since thermal conditions and, in particular, HVAC systems signify an important energy use in buildings, one of the most important objectives of the CEU Team was to design and implement an efficient solution for the SMLsystem prototype. In this paper the technical aspects of the SMLsystem regarding its thermal comfort conditions are sequentially outlined as well as the basis of the HVAC design and its detailed description and the analyzed results of the proposed system. 1.1. SMLsystem technical description SMLsystem was born from the investigation of the House SMLhouse (small, medium and large) proposed in the Solar Decathlon 2010 Edition. SMLsystem returns to prefabrication as a starting point to respond to the new ways of inhabiting. The challenge of the SML system proposal consists in defining an architectural language where structural, composition and functional values are introduced in a coherent way (see Fig. 2). It allows the user to configure the space according to their needs by making a catalogue

http://dx.doi.org/10.1016/j.enbuild.2014.04.029 0378-7788/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: A. Real, et al., Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation, Energy Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.04.029

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Fig. 1. Solar Decathlon Europe 2012 (left) and SMLsystem (right).

of prefabricated elements available. The unit or basic module is formed totally by prefabricated materials and dry-assembled, being wood the predominant material in the SML system, to create an entirely prefabricated module. Each of these units is transported fully equipped, only lacking the joints of the systems and its own assembly, taking special care of tightness and thermal bridges, an aspect studied from the initial design. SMLsystem is mainly made in wood; specifically it works with CLT (cross-laminated timber) panels, acting as structural elements, even in horizontal or vertical plan. The module is formed by a laminated pine wood structure, ventilated fac¸ade in wood with composed closure (wood + insulation + wood + air camera + double gypsum board), and a vertical lattice system where larch wood and ceramics are used in order to get different thermal inertia and to be able to use them depending on the external conditions. This system is dynamic, and allows the owner to create transition spaces between exterior and interior, just expanding them into the longitudinal module axis, at the same time that provides solar protection to the openings. The courtyard acts like a composition element and conditioning of the housing; in all constructive units (base modules) there is a courtyard that divides the space, generating the access of the house, offering lighting and ventilation control and a great space value. The inclusion of the courtyard is not part of the constructive SMLsystem unit as an element, but it is projected in guidance and relationship between the modules. The patio will vary its length, being an extension from the fac¸ade. In our proposal, the courtyards play an important role in air conditioning, favouring cross ventilation in the house’s interior acting as a passive system of energy-saving and using the fac¸ades horizontal and vertical louvres, increases its effectiveness. This will reduce the excess radiation at certain times of the day, as well as shade and offer more privacy to the housing.

Moreover, the prototype comprises mostly of wood, which besides being a fully recyclable material, allows us to drastically reduce the weight of the building. The engineering facilities have been distributed in two main places: the Engineering Box located outside the house (allowing its modular design and flexibility) and the technical room located inside. The Engineering Box is a room of approximately 1 × 13 m and 1.5 m high. It is conceived as a modular installations room as each part can be designed separately and then assembled together on site. Our aim was to design a “plug and play” facility according to the competition. It contains the domotic and plumbing elements, water supply tank, the PV installation and the HVAC system. It is linked to the house by means of an under floor connection. This connection enters the house through the floor of the technical room, which also hosts the DHW and ventilation. The ventilation system is designed to control the CO2 level of the house. It extracts the air from the bathroom and the kitchen and renewed air is driven in the bedroom and living room. A heat exchanger has been installed to recover up to 92% of the thermal energy of the exhaust air. Thus we spare the energy needed to heat or cold the renewed air. The DHW system consists of an accumulator linked to two ultra high vacuum solar panels installed on the roof. These panels have a high efficiency even on cloudy days. The annual coverage of the DHW system is 71% according to calculations. All the solar panels were set in a pack that is placed on the top of each module. As stated before, reducing energy demand is one of the main objectives of SMLsystem. The main challenge of energy efficiency is to reduce power consumption without affecting the comfort conditions. Passive systems incorporated into its design minimize the need for and dependence on active systems to cool and heat the house. In other hand, efficiency of active systems optimizes the use of energy and reduces demand. For the HVAC system, a heat pump

Fig. 2. Floor plan and render of the SMLsystem.

Please cite this article in press as: A. Real, et al., Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation, Energy Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.04.029

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Fig. 3. Functioning schematic of the heat storage tank for dissipation.

combined with a Thermal Energy Storage (TES) system has been chosen. The operation of the HVAC system for the optimization of consumption compared to generation was linked to a predictive domotic system described in other work (Towards energy efficiency related to indoor temperature forecasting using multivariate analysis, Zamora, et al.). 2. Background and objectives HVAC systems represent an important percentage of the energy consume in buildings. Besides, in warm countries such as Spain, most of this energy is used for cooling during the summer months. This cold energy is commonly supplied with systems based in vapour compression cycles. The performance of these systems is strongly dependent on the temperatures of the cold and hot reservoirs. When cooling, the cold reservoir is the building itself and its setpoint temperature is controlled by the user. The hot reservoir is the outside of the building and the temperature cannot be controlled. The higher the outside temperature the lower the efficiency of the system and, as the cold demand is higher for high outside temperatures, the system is forced to work most of the time at a low efficiency ratio. This paper exposes a HVAC system installed and tested during the Solar Decathlon Europe 2012 contest, which aims to improve the performance of a heat pump by means of a thermal storage system using phase change materials (PCM). PCM are commonly used in construction as a passive system for thermal storage. Bricks and fac¸ades combined with PCM provide a constant temperature in the inside of the building [1]. PCM are also combined with heat pump HVAC systems to take advantage of the low outside temperatures at night to cool a heat storage tank with a high COP [2,3]. This tank is later used to cool the building when the outside temperature rises. These systems, as well as PCM storage systems for ventilation, have been used and tested in energy efficient houses in Solar Decathlon contest in both the European and the American edition [4]. A disadvantage of such systems is the need to predict the demand as, if the energy is stored for long periods of time, losses can reduce the efficiency of the system. The system proposed in this paper is a heat pump based HVAC system with two thermal storage tanks: a cold tank and a warm tank. The cold tank provides the possibility to store energy at night as described above. The warm tank is a more flexible option to improve the efficiency of the heat pump. This tank works as an alternative hot reservoir. If the outside temperature is high the heat pump can dissipate the heat in the warm PCM tank at a constant temperature. The heat stored during the day in the tank can be rejected to the outside at night when the temperature drops under a certain limit (see Fig. 3). This use of thermal energy storage has some advantages over conventional cold energy storage. First of all, there is no need to predict the demand as no energy is stored to cover future demands. Cold energy is generated in the instant it is demanded and the heat stored is the waste of the process. This represents a second

Fig. 4. Temperature intervals for the contest and points earned (from SDE2012 rules).

advantage: as the energy stored is a waste, energy losses of the storage tank are not as troubling. But this system must not only be efficient but met the Solar Decathlon comfort conditions above mentioned. The comfort conditions of the house are constantly monitored and evaluated. Humidity, CO2 level and temperature of the house must be kept between established limits. Particularly, the temperature of the dinning room and the bedroom must remain between 23 and 25 ◦ C to gain all the points. Reduced points are earned if the temperature remains between 20 and 23 ◦ C or between 25 and 28 ◦ C (see Fig. 4). The temperatures are simultaneously measured in both the bedroom and the dinning room and both must be kept between the limits, so the HVAC system must provide an homogeneous climate. Also the range of temperatures is very narrow, therefore the HVAC system must provide a very accurate control of temperatures. This could entail an intermittent performance of the heat pump which is negative for its efficiency and for the lifespan of the components. The cold storage tank will grant a more steady performance of the heat pump as explained in the next sections. 3. System and components’ description 3.1. Thermal load of the SMLsystem The first step to dimension the HVAC system is to evaluate the thermal load to cover. The HVAC equipment of the SMLsystem has been conceived for its use during the contest Solar Decathlon Europe 2012. Therefore the thermal load has been calculated for Madrid during September and during the whole year. The weather data for the contest placement have been obtained using the software Meteonor. We have considered the mean temperatures of the period 1996–2005. Regarding the thermal loads generated inside the house we have considered two users, one television, one computer and one fridge. To calculate the thermal loads a 3D model of the house was developed using Google SketchUp. After including all the shading factors and the thermal properties of the enclosures, the software

Please cite this article in press as: A. Real, et al., Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation, Energy Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.04.029

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Fig. 5. Monthly heating and cooling thermal demand for normal usage conditions.

Fig. 6. Monthly heating and cooling maximal load for normal usage conditions.

Trnsys 17 and Trnbuild have been used to calculate the internal parameters of the house (temperature, humidity, etc) as well as the energy input necessary to keep the comfort conditions. According to these considerations, two different simulations have been carried out: a less demanding annual simulation, where the temperature of the house is kept between 20 and 26 ◦ C and a more demanding simulation during the two contest weeks where the temperature of the house is forced to remain between 23.5 and 24.5 ◦ C. Fig. 5 shows the monthly thermal demand simulation for heating and cooling. Fig. 6 shows the peak thermal load for each month. The maximal load takes place in July with 2011 W. According to these calculations a 2 kW heat pump would be adequate for a normal usage during the year. But the HVAC equipment of the SMLsystem has been designed for the SDE2012 contest with more demanding thermal conditions. Therefore, another simulation has been carried out for the two contest weeks. Fig. 7 shows the daily thermal demand simulation for heating and cooling. Fig. 8 shows the peak thermal load for each day. According to calculations we should expect a peak load of 3823 W.

Thus we need a heat pump of, at least, 4 kW to cover the expected thermal demand during the contest. 3.2. Heat pump As there is not a wide range of water/water heat pumps for such a low thermal power we selected the heat pump with the next higher thermal power available: Terra 7 S/W from the Austrian company IDM. This heat pump provides 6.76 kW with a COP of 4.40 for S0 ◦ C/W35 ◦ C according to EN 255. 3.3. Phase change material (PCM) There is a wide variety of phase change materials depending of their many uses. For our HVAC system we needed a PCM with a phase change temperature that fits our requirements i.e. 27 and 10 ◦ C. Also the PCM container should be easy to stack in our tanks. According to these requirements the PCM product chosen were S10 and S27 from the company PCMproducts Ltd. a hydrated salt

Fig. 7. Daily heating and cooling thermal demand for contest conditions.

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Fig. 8. Daily heating and cooling maximal load for contest conditions.

Fig. 9. FlatICE container (left) and heat storage tanks (right).

based products with phase change temperatures of 10 and 27 ◦ C respectively. These materials are stored in FlatICE containers which are easy to stack in height (see Fig. 9). 3.4. Fan and radiators As described above, during the contest the temperature is measured in two different places of the house and both temperatures must remain between the temperature limits. For that reason an homogeneous climate of the house is needed. A supply system with a large thermal exchange surface would provide us an homogeneous climate of the house. Also, this large surface would allow us to use higher supply temperatures thus increasing the COP of the heat pump. We selected supply and return temperatures of 12 and 16 ◦ C respectively compared to normal 7–12 ◦ C operation. The Clima Canal radiators from the Belgian company Jaga have been selected (see Fig. 10). The Clima Canal is a micro finned radiator designed to be installed on the floor. 9 m of Clima Canal radiator have been installed in the SMLsystem in order to grant the temperature homogeneity necessary for the contest. The fan selected to reject the heat is the model AIC 63 from the company Heatsun. It has an electrical power consumption of 200 W and a thermal removal capacity of 42.450 kcal/h. 4. Schematic and functioning modes The HVAC system is based on a conventional water to water heat pump which works between the house and a fan (thermal rejecter). Furthermore, two thermal storage tanks are provided: a cold tank and a warm tank. The tanks are filled with containers of phase change materials (PCM). These PCM are a mixture of water with salts with a melting point temperature different from zero. Thereby we can take profit of the latent heat of the PCM at

Fig. 10. Radiador Clima Canal.

temperatures which provide a good performance of the heat pump. According to expected thermal conditions of Madrid during the weeks of competition 27 ◦ C PCM has been selected for the warm tank and 10 ◦ C PCM for the cold tank. The main purpose of the warm tank is to offer the heat pump a sink to reject the heat at constant temperature when the outside temperature is high. Meanwhile, the cold tank has two objectives. If the outside temperature is low at night and cold demand is expected for the house the following day, the heat pump can cool the tank at night with a high COP so the tank can later cool the house without using the heat pump. The second objective of the cold tank is to prevent a more steady performance of the heat pump. If the heat pump is cooling the house and the target temperature is reached, in stead of turning off the heat pump until the temperature rises again, it will start cooling the cold tank so when the house’s temperature rises the thermal demand can be covered with the tank. Thus the heat pump works for longer periods and more steady extending the life of the components. With these four possible destinations for the heat pump up to seven different functioning modes are available (see Table 1). When the temperature of the house needs to be reduced, the first option is always the cold tank in case it has been previously cooled: Mode 1. The thermal energy stored in the cold tank should be used as soon as possible to prevent losses which will reduce the global performance of the system. If cold demand cannot be covered with the cold tank it must be covered with the heat pump. In this case there are two possible functioning modes: Modes 2 and 3. If the outside temperature is below a certain limit, the heat can be rejected outside using the fan granting the system an admissible COP. If the outside temperature is above that limit, the heat will be rejected to the warm tank

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6 Table 1 Description of the functioning modes. Mode

Scheme

1

cold tank

house

2

dissipator

heat pump

house

3

warm tank

heat pump

house

4

dissipator

warm tank

5

dissipator

heat pump

cold tank

6

warm tank

heat pump

cold tank

7

dissipator

heat pump

which provides a heat sink at a constant temperature granting thus a minimum COP for the HVAC system. The energy accumulated in the tank in Mode 3 must be rejected later when the outside temperature drops below a certain limit. In this case the system works in Mode 4 connecting the warm tank with the fan to reject the heat outside. If the cold tank must be cooled, there are two different heat sinks. In the Mode 5 the cold tank is connected to the fan to reject the heat outside while in Mode 6 it is connected to the warm tank. If the there is a heat demand in the house, the system works in Mode 7. Only one configuration has been considered to supply heat. The heat pump is connected to the house and to the fan. The system should be configured to provide an easy alternation between modes with the less components and the more simple installation possible. Given these premises a layout was designed based on two main water circuits connected to the heat pump and to each of the four possible destinations (see Figs. 11 and 12). . A total of 13 electrovalves are needed to stablish the seven different configurations. The circuit needs three circulating pumps, one of them already included inside the heat pump. Each valve has

house

been named as shown in Fig. 11. The circulation pumps have been named as PX where X stands for the number of the pump. Table 2 represent a brief description of the state of each component of the system to configure each one of the seven modes described above. A Mode 0 has been defined where none of the seven modes is active, for example when there is no demand and no accumulation or dissipation is needed in any storage tank. In this table the column “C” stands for Compressor. The column “Inv” stands for Inversion as the heat pump can invert its functioning mode. In a normal functioning mode (Inversion off) P1 supplies hot water and P2 supplies cold water. In an inverted functioning mode (Inversion on) P1 supplies cold water and P2 supplies hot water. In this table an “X” means “closed” for valves and “off” for the rest of components. A “O” means “open” for valves and “on” for the rest of components. 5. Simulations and expected savings To evaluate the benefits of this HVAC system a basic simulation has been carried out to be improved and tested with real

Please cite this article in press as: A. Real, et al., Improvement of a heat pump based HVAC system with PCM thermal storage for cold accumulation and heat dissipation, Energy Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.04.029

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Fig. 11. System schematic with valve and pump numeration.

Fig. 12. Flow diagram for the cold function.

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8 Table 2 System configuration for each functioning mode.

Fig. 13. COP for the heat pump with and without warm tank. Fig. 14. Daily electrical consumption for the heat pump with and without warm tank.

measurements. This simulation shows the yearly electrical consume for a conventional heat pump system and for a heat pump system with heat thermal storage. Only the cold supply periods have been considered as the tanks are not used when supplying heat. Fig. 13 shows the COP of the heat pump working in both considered modes throughout the year. The high values of the COP are due to the supply temperature which has been set to 12–16 ◦ C compared to normal 7–12 ◦ C operation. Besides it only considers the consume of the compressor. In the first graph the COP of the heat pump varies with the outside temperature, thus in the summer months (hours 4000–5500) the COP reaches minimum values around 3. The second graph shows the COP of the system using the warm tank. As can be seen the COP has a bottom value which is rarely crossed. Only when the heat stored in the tank cannot be rejected due to high night temperatures during several days the system is forced to work in the conventional mode rejecting the heat to the outside using the fan. During these periods the system has the same COP as the conventional mode. The improvement of the COP reduces the electric consumption of the compressor and thus the consumption of the whole HVAC system. However, the use of the warm tank implies the use of the circulation pump twice: first to dissipate in the tank and secondly to dissipate with the fan at night. The electrical savings on the compressor must be higher than the increased consume of the circulation pump to increase the efficiency of the whole system.

Fig. 15. Daily electrical energy savings using the warm tank.

Fig. 14 shows the daily overall electrical consumption of both HVAC systems. This consumption includes the compressor, the circulation pumps and the fan. The difference between both consumes represent the savings of using the warm tank. These savings are shown in Fig. 15. According to this simulation the electrical consumption for cold supply in the conventional functioning mode rises to 685 kWh. With the use of the warm tank the electrical consumption calculated was 555 kWh which represents an 18.97% energy savings. 6. The experience during the contest. Performance of the HVAC system During competition pressure problems in the tanks forced the system to work only with the modes 2 and 7, this is the

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conventional heat pump functioning, dissipating the heat in the fan. For that reason the savings calculated with the simulations exposed in the previous section could not be verified with real measurements during the contest. However, during the temperature measurement periods, when the HVAC system was connected, the temperature was successfully kept within the established limits with very few difference between the dinning room and the bedroom temperatures.

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Although energy savings of 18.97% were obtained with simulations, only partial real measurements could be obtained during competition due to technical problems. Since the prototype is already functioning in the University campus after competing in SDE2012, new experimental data and measurements are being obtained in order to compare numerical and operating conditions in normal functioning applications. References

7. Conclusions The PCM storage tanks of the HVAC system designed for the SMLsystem allow the COP to be independent of the outside temperature granting the efficiency to be, most of the time, over a bottom value. Simulations have been carried out to evaluate the energy savings obtained with the use of the warm tank as a dissipation reservoir. The HVAC system covered the demand of the house during the Solar Decathlon competition and successfully kept the temperature within the established limits.

[1] A. de Gracia, et al., Experimental study of a ventilated fac¸ade with PCM during winter period, Energy and Buildings 58 (2013) 324–332. [2] F. Agyenim, N. Hewitt, The development of a finned phase change material (PCM) storage system to take advantage of off-peak electricity tariff for improvement in cost of heat pump operation, Energy and Buildings 42 (2010) 1552–1560. [3] J. Long, D. Zhu, Numerical and experimental study on heat pump water heater with PCM for thermal storage, Energy and Buildings 40 (2008) 666–672. [4] E. Rodriguez-Ubinasa, L. Ruiz-Valeroa, S. Vega, J. Neila, Applications of phase change material in highly energy-efficient houses, Energy and Buildings 50 (2012) 49–62.

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