Scholarly article on topic 'BIPV/T+ASHP: Technologies for NZEBs'

BIPV/T+ASHP: Technologies for NZEBs Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Raghad Kamel, Navid Ekrami, Peter Dash, Alan Fung, Getu Hailu

Abstract A full-scale test facility of building Integrated Photovoltaic/Thermal (BIPV/T) collector coupled with cold climate variable capacity Air Source Heat Pump (ASHP) and Thermal Energy Storage (TES) was designed to be implemented at Toronto and Region Conservation Authority (TRCA)’s Kortright Centre. The PV/T array consists of 25 panels. The warm air generated in the BIPV/T array is considered the source of the heat pump for thermal energy production. Coupling of BIPV/T and ASHP enables a highly efficient heating system in harsh winter conditions. Thermal energy from PV/T array could be stored in the TES (concrete slab or gravel bed beneath the floor) during day and released in night time to enhance the performance of the heat pump. It is shown that using air thermal storage to preheat the outdoor air as an inlet flow to the air source heat pump increases the coefficient of performance (COP) of the heat pump. Consequently, electricity consumption by the ASHP decreases during night. Analytical and numerical methods are used to evaluate design parameters that influence thermal energy production, electrical energy production, heat pump COP and electricity consumed by the heat pump. Moreover, a sensitivity analysis was conducted to optimize water storage tank size, assuming that the heat pump would only operate during hours of thermal generation from the PV/T array. The preliminary results show that the seasonal COP could be increased from 2.74 to a maximum value of 3.45 for direct coupling of BIPV/T+ASHP without the use of diurnal thermal storage. The heat pump electricity consumption is reduced by 20% for winter.

Academic research paper on topic "BIPV/T+ASHP: Technologies for NZEBs"

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Energy Procedia 78 (2015) 424 - 429

6th International Building Physics Conference, IBPC 2015

BIPV/T+ASHP: Technologies for NZEBs

Raghad Kamela*, Navid Ekramia, Peter Dasha, Alan Funga, Getu Hailub

aRyerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada bUniversity of Alaska Anchorage, 3211 Providence Dr, Anchorage, AK 99508, United State

Abstract

A full-scale test facility of building Integrated Photovoltaic/Thermal (BIPV/T) collector coupled with cold climate variable capacity Air Source Heat Pump (ASHP) and Thermal Energy Storage (TES) was designed to be implemented at Toronto and Region Conservation Authority (TRCA)'s Kortright Centre. The PV/T array consists of 25 panels. The warm air generated in the BIPV/T array is considered the source of the heat pump for thermal energy production. Coupling of BIPV/T and ASHP enables a highly efficient heating system in harsh winter conditions. Thermal energy from PV/T array could be stored in the TES (concrete slab or gravel bed beneath the floor) during day and released in night time to enhance the performance of the heat pump. It is shown that using air thermal storage to preheat the outdoor air as an inlet flow to the air source heat pump increases the coefficient of performance (COP) of the heat pump. Consequently, electricity consumption by the ASHP decreases during night. Analytical and numerical methods are used to evaluate design parameters that influence thermal energy production, electrical energy production, heat pump COP and electricity consumed by the heat pump. Moreover, a sensitivity analysis was conducted to optimize water storage tank size, assuming that the heat pump would only operate during hours of thermal generation from the PV/T array. The preliminary results show that the seasonal COP could be increased from 2.74 to a maximum value of 3.45 for direct coupling of BIPV/T+ASHP without the use of diurnal thermal storage. The heat pump electricity consumption is reduced by 20% for winter.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunderresponsibilityof theCENTROCONGRESSIINTERNAZIONALESRL Keywords: PV/T system; thermal energy storage; heat pump

* Corresponding author. Tel.: +1-416-979-5000 Ext. 7833 E-mail address: raghad.kamel@ryerson.ca

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL doi:10.1016/j.egypro.2015.11.687

1. Introduction

The residential sector in Canada consumes energy mostly for space heating (63%) and domestic hot water (17%) [1].The energy required by the world is estimated to increase by 50% from 2005 to 2030 due to population growth and economic development [2]. Significant energy and cost savings could be achievable by integrating sustainable and state of the art renewable technologies in one combined system [3]. Buildings that utilize highly efficient mechanical systems combined with solar thermal systems, along with a high performance building envelop, to produce as much energy as it consumes over the course of the year are termed Net Zero Energy Buildings (NZEBs).

The PV/T system collects energy at low temperatures and may not work efficiently in direct heating. This energy could be used as a source to the heat pump in winter [4, 5]. In other words, the solar collector system provides energy with temperature above the ambient temperature, improving the COP of the heat pump. So, integrated solar energy systems with heat pump is an interesting field of research since it provides saving in energy more than that from each individual system [6]. The electricity needed for operating the heat pump system from the external network is reduced by the direct use of electricity produced by the PV array within the combined PVT and heat pump system. The combined solar assisted heat-pump system appears to be a suitable alternative, which not only saves building space, but also reduces the reliance on electricity utilities supply. The combined system is then able to work more efficiently.

Few studies investigate the possibilities of combining a solar system with air source heat pump. One of the advantages of air source heat pumps is that their cost is much lower than GSHP [7].Moreover, some ASHPs types can deliver heating and cooling at the same time, in effect transferring heat from the warmer area in the building (such as a south-facing room) to a colder zone in the building. Some installers put the ASHP in a covered "penthouse" near the building exhaust openings to utilize any waste heat [8]. Since most of the energy collected by the solar thermal collectors is available during periods with lower thermal load. Therefore, Badescu [9, 10] examined the benefit of using air thermal storage with the combined solar collector and heat pump for heating purposes. Considering that the heat demand is at maximum in winter, or at night, when the supply of solar energy is minimal or zero. This makes the thermal storage an essential part of a solar powered heat pump system. The operation of a TES unit is similar to a common heat exchanger. However, the TES unit is either being charged or discharged at a given time, so that its operation is essentially unsteady.

This paper describes the design of a full-scale test facility of Building Integrated Photovoltaic/Thermal (BIPV/T) collector coupled with cold climate Air Source Heat Pump and Thermal Energy Storage systems. The test facility is currently under construction phase at Toronto and Region Conservation Authority (TRCA)'s Kortright Centre. The goal of the project is to achieve the near net-zero annual energy consumption. Numerical analysis and simulation tools were used to characterize the energy performance of the PV/T systems with various design variables and to develop thermal storage system (hollow core concrete slab). An analysis was conducted to optimize the size of water-PCM based diurnal thermal storage for air-to-water heat pump option.

2. Test hut facility description

The test facility is currently under construction at Toronto and Region Conservation Authority (TRCA)'s Kortright Centre. The goal of the test facility is to investigate the performance of combining passive system and dynamic building envelope technologies such as hybrid solar systems, short-term thermal storage and heat pump to achieve near net-zero annual energy consumption. The test facility (9.14m x 7.62m x 3.2m) is split into two conditioned zones and a mechanical room. The envelope of the house, such as the walls, can also be used as a thermal storage. Insulated Concrete Forms (ICF) are modules used to build walls, and are more efficient than conventional walls. ICF walls are constructed by assembling concrete between polystyrene foams. The foam could be made with polyurethane, recycled wood and cement mixture. The commonly used foam is either Polystyrene with RSI value of 0.26 per cm or extruded polystyrene with the RSI value of 0.35 per cm [11]. Also, American Society for Testing and Material regulated an international standard for ICF walls to control the quality of concrete, width of wall, maximum aggregate size, and minimum compressive strength of concrete.

The mechanical systems of the test facility can be categorized into three main parts:

- Building Integrated Photovoltaic/Thermal collector (BIPV/T) system

- Heat Pump (HP) system

Fig 1. Schematic of BIPV/T system integrated with ASHP and thermal energy storage system.

- Thermal Energy Storage (TES) system including ICF walls, Ventilated Concrete Slab, Gravel/Sand bed, and Water tank storage

3. BIPV/T system and heat pump

The design integrates a 6.5 kWp building integrated photovoltaic/thermal (BIPV/T) system on its south-facing roof. The BIPV/T system uses outdoor air circulating in an air channel beneath the PV panels in order to recover heat from them, thus lowering their temperature. Building integrated photovoltaic/thermal system means that the system is a part of the roof, in other words, the lower channel material is the roof itself with RSI 7.

The preliminary analysis was done based on the verified model of the Archetype Sustainable House (ASH) at Kortright Centre. The heating loads for the test hut will be manipulated to match that of the ASH. A TRNSYS model of the House was developed and validated with the data collected from

monitoring the ASHP by Safa et al. [12]. The model was modified by integrating a PV/T system on the south-oriented roof. Then, the PV/T array was linked to the heat pump. The analysis has been conducted to estimate the potential benefit and improvement in the heat pump performance and potential reduction in electricity demand for combining PV/T system with variable capacity air-source heat pump at the ASH [13, 14]. The results showed that increasing air mass flow rate leads to more heat recovered from the PV panels. For the array of 5 x 5 PV/T panels (5 rows, each row has 5 panels), the generated thermal energy grew dramatically until the total mass flow rate reached 5 kg/s. As the mass flow rate becomes very large, the temperature rise from inlet to outlet decreases toward zero, reducing the improvement of the COP of the heat pump. Low air flow rate is preferred to boost the heat pump COP but, on the other hand, the combined system needs enough air flow rate in the source side of the heat pump, which means adequate thermal energy to be absorbed by the evaporator in heating season. This point should be considered to maximize the performance of the whole system. Therefore, air mass flow rate through the PV/T collectors should match the required air flow over the heat pump outdoor coil according to the manufacturer manual. It was found that for the combined system (PV/T +ASHP) the heat pump cumulative electricity consumption (day and night) for a typical heating season (from October 1 to May 22) could be reduced by 20.2%. The seasonal COP could be increased from 2.74 to a maximum value of 3.45 for direct coupling of BIPV/T+ASHP without the use of diurnal thermal storage. There is flexibility for such an integrated system to work at night and day to provide heating and cooling. In this case, the operation conditions will not be affected by solar irradiance intensity. However, the challenges for coupling an air based PV/T system with an air source heat pump are related to the design of the control system, which should work optimally under all conditions.

Table 1. Different operation options of the ASHP for the test facility project

Winter Summer All year

Load Outdoor coil (source) X ? I i X < X 1 Desuperheater

AHU (cooling mode) \ X X

AHU (heating mode) X , > i

DHW(HX) X X X X

In floor heating (HX)

Work individually according to priory, use thermal energy from PVT system Use thermal energy collected from PV/T array 10% of total capacity

It is essential to develop proper control strategies, which will enable the system to work efficiently under different weather conditions, with a particular focus on combined space heating and cooling, and domestic hot water production. In addition, the ducting design of the combined system itself, i.e., connecting PV/T system with heat pump, is another challenge. Figure 1 shows a schematic of such an integration. It is suggested to use a louver box with dampers to direct air flow, either to the heat pump or to the air thermal storage. In winter, the source of the heat pump is the air from PVT during sunny hours or from air thermal storage during night, while during summer the heat pump works to cool the space. In this case, the collected thermal energy from the PV/T system is used to produce DHW. For such integration, a custom designed heat pump is needed to satisfy the required functions (cooling in summer, heating in winter, and production of domestic hot water during the entire year). In cooperation with local industrial partner [15], an air source heat pump is currently being designed to be coupled with a PVT system while operating in different modes. Table 1 shows the different operation modes of the ASHP for the test hut facility project.

4. Thermal Energy Storage Systems

As part of the optimization of the design of these thermal storage systems, the entire building has to be designed as multiple efficient thermal storage systems. The design approach must be in a way that the house stores energy as much as possible from the sun during the day. Therefore, different components of the house potentially can be used as storage systems. Consequently, connecting all three major parts of the system to each other require a complicated duct/pipe design and installation. Figure 2 shows the simplified model of combined roof mounted PV/T, heat pump, and ventilated gravel/concrete bed.

4.1. Insulated Concrete Forms (ICF) Walls

As mentioned earlier, besides the structural function of the concrete wall, it can also serve as actively charged thermal mass to store thermal energy and then passively release it to assist space heating. However, in this project the objective of using an ICF wall as TES is not to discharge the thermal energy to the space directly, rather to re-use it as an input of another mechanical system such as heat pump or re-use it through the hydronic forced air or infloor heating. The ICF walls are designed to be charged with heated Fig 2. Test Facility Cross-^™ View

water provided by an air to water heat pump when the sun is available and the solar assisted heat pump works in more efficient manner. Stored thermal energy inside the concrete can be used to heat up the water in a later time when there is a demand. However, the thermal energy of the water can also be discharged directly to the house according to the demand.

A comprehensive study on thermal behaviour of ICF walls has been done using Flow Simulation package of Solidworks software. Thermal/physical aspects of the ICF wall were fully analysed to investigate the optimal design and gain detail understanding of heat transmission between water in the pipes and concrete. In this study a simplified 3D model of unit length (1 meter) PVC pipe embeded inside concrete was developed. The dimensions for the model are: a pipe with 1.7cm inside diameter, 2.13cm outside diameter (0.23cm thickness) and 1 meter length. The concrete wall thickness was 15.24cm. The physical properties of the insulating foam are not considered. Instead, the walls are considered adiabatic. Different temperatures and velocity scenarios were tested and maximum distance between parallel pipes for a uniform thermal distribution and efficient use of concrete was found to be 50.8cm, while was recommended to set the pipes distance at 75% of the maximum allowance for a more efficient design.

Three different types of concrete such as Lightweight, Medium, and Dense concrete were initially examined and Medium concrete had been chosen for the simulation because of its thermal and physical properties. The density of 2000 kg/m3, specific heat of 1000 J/kg.K, and thermal conductivity of 1.13 W/m.K was considered for the simulation.

Five different inlet temperatures (30°C, 35°C, 40°C, 45°C, and 50°C) were tested as possible range of temperatures produced by heat sources for hot water production. Velocity plays an important role in the heat transfer rate. Therefore, four velocities with one laminar (0.01m/s) and three turbulent (0.1m/s, 0.5m/s and 1m/s) flow were considered to show a variation of heat transmission process between pipes and concrete. Temperature of the outer walls were fixed at 20°C and inlet and outlet effect were considered. Analyizing the overall performance of the system requires to investigate the heat transfer rate between the water and concrete for each of the temperature-velocity settings. Since

the transient behavior of the system is important, heat transfer rate of the system was studied for each time step (250 seconds) of the simulation for total 24 hours. Initialy, the heat transfer rate is higher for all the temperature-velocity settings. It is due to high temperature difference between the hot water and concrete. As time passes, the temperature inside the concrete increases and heat transfer rate decreases. Sample calculations for two inlet temperatures are presented in Table 2, which presents the total amount and average rate of energy transferred from water to the concrete.

Table 2. Simulated Results of Different Temperature-Velocity Settings for one meter length of Pipe

Temperature of Velocity of the fluid Total Transferred thermal Average heat transfer rate

the fluid (°C) (m/s) energy after 24 hours (Wh) (W)

0.01 (Laminar) 260.35 10.89

0.1 (Turbulent) 294.70 12.33

40 0.5 (Turbulent) 308.15 12.89

1 (Turbulent) 309.48 12.94

0.01 (Laminar) 390.96 16.35

0.1 (Turbulent) 451.22 18.88

50 0.5 (Turbulent) 462.46 19.35

1 (Turbulent) 464.37 19.43

4.2. Ventilated Concrete Slab (VCS) & Ventilated Gravel / Sand Bed

The floor and foundation of the test facility is designed to perform as an Air Based Thermal Energy Storage (TES) system. Stored thermal energy during sunny hours can be used to increase the inlet air temperature for the ASHP during night time. This configuration is expected to enhance the overall performance of the integrated system by implementing the TES. The VCS and Gravel/Sand Bed are designed to be charged with solar-heated outdoor air in an open loop configuration. It is based on commonly available construction technologies and can thus be easily implemented in North American homes. Garage slab as an unconditioned space or floor slab as conditioned slab are different options that can be considered for houses. A corrugated steel deck can be easily placed under the floor concrete during the construction and spaces between the edges can be used as air channels through the slab. Also, 10 cm nominal diameter air pipes will be installed for the sand bed in the east zone of the building while west zone is designed for passing air through gravel with no pipes.

4.3. BIPV/T +ASHP Water and Phase Change Material Storage

A sensitivity analysis was conducted on water tank sizes as diurnal thermal energy storage. The goal of the sensitivity analysis was to optimize the storage tank so that the heat pump would only operate during hours of thermal generation from the BIPV/T. It was found that as storage size increased above a certain size, further increase in storage size resulted in marginal increase in storage capabilities. The storage tank was modelled with and without phase change material (PCM) with a phase change temperature of 46°C. The PCM's only contribution to the energy storage was assumed to be its latent heat of fusion. To model an air-to-water VCASHP, the experimentally validated air-to-air VCASHP by Safa et al. [16] was modified by shifting the COP curve of the heat pump 10°C higher to achieve a higher outlet temperature from the heat pump. The set point temperature of the storage tank was 50°C. If the heat pump was not able to keep the tank above 30°C, it was assumed that a back-up system was used for heating. The tank was perfectly insulated and well-mixed. The storage tank was modelled on an hourly basis. Table 3 shows the percent of the heating demand met by the various storage options considered. A 500 L, 1000 L, and 1500 L tank were considered and filled with 0%, 10%, and 40% PCM by volume. The demand was not met when the storage tank temperature dropped to thirty degrees Celsius as that temperature was considered insufficient to provide heating to the house. When the heat pump was allowed to operate at night, the storage size did not dramatically affect the percent of demand met, as the heat pump was always available for heating. However, when the heat pump was restricted to operate when the BIPV/T was generating thermal energy, the size and medium of the storage tank was very important. The heat pump's output could be increased by optimizing the compressor operation of the heat pump.

Table 3. Seasonal demand met by storage options considered for air-to-water heat pump

Storage medium Tank size (L) Demand met over heating season with heat pump operating when required Demand met over heating season with heat pump restricted to operation with BIPV/T thermal production

Water 500 79% 46%

Water + PCM (10%) 500 79% 50%

Water + PCM (40%) 500 81% 60%

Water 1000 80% 56%

Water + PCM (10%) 1000 81% 61%

Water + PCM (40%) 1000 83% 64%

Water 1500 82% 61%

Water + PCM (10%) 1500 82% 63%

Water + PCM (40%) 1500 84% 65%

Conclusion

Preliminary analysis shows that an integrated BIPV/T+ ASHP +TES increase the overall COP of the system. Therefore, this system is expected to reduce the operational cost. Using Thermal Energy Storage (TES) systems in residential buildings helps to store thermal energy when it is available and be released when it is needed. This will improve the energy efficiency of the buildings. Such integrated system can bring existing and new buildings closer to

the goal of net-zero energy status in highly cost-effective and environmentally friendly manner. Acknowledgements

This project is supported by Toronto Atmospheric Fund (TAF), MITACS, Natural Sciences and Engineering Research Council of Canada (NSERC), Smart Net-Zero Energy Buildings Research Network (SNEBRN), Toronto and Region Conservation Authority (TRCA), and Ontario Graduate Scholarship (OGS).

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