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<ul><li><p>2nd Canadian Solar Buildings Conference Calgary, June 10 14, 2007 </p><p> 1</p><p>DESIGN AND SIMULATION OF A BUILDING INTEGRATED PHOTOVOLTAIC-THERMAL SYSTEM AND THERMAL STORAGE FOR A SOLAR HOUSE </p><p> YuXiang Chen1*, A. K. Athienitis1, B. Berneche2, Y. Poissant3, K. E. Galal1 </p><p>1Department of Building, Civil and Environmental Engineering, Concordia University 1515 St. Catherine W., EV-6.139, Montral, Qubec, Canada, H3G 2W1 </p><p>*Tel: (514)848-2424 Ext. 7080 email: 2 Alouette Homes, 200 rue Des Alouettes, St-Alphonse-de-Granby, QC J0E 2A0 3 CETC Varennes PV &amp; Hybrid Systems Program, Natural Resources Canada </p><p>ABSTRACT This paper describes the design and simulation of a building integrated photovoltaic-thermal system with heat recovery and storage for a solar house. This solar house is to be built by Alouette Homes (AH), a prefabricated-home manufacturer, as its project for Canadas EQuilibrium Housing demonstration initiative. </p><p>The design of the building integrated photovoltaic-thermal (BIPV/T) system and ventilated concrete slab thermal storage system, which use air heated by BIPV/T as heat source, will be discussed as one of many possible and feasible ways for maximizing solar energy utilization. The BIPV/T system can harvest a considerable amount of useful heat; however, some of this energy typically needs to be stored for later use (e.g. at night) with an appropriate thermal storage design. A hollow core concrete thermal storage system is utilized in addition to hot water and direct gain thermal mass. Simulation results are presented from a transient finite difference model for the house, including the BIPV/T system and ventilated concrete slab. </p><p>INTRODUCTION In fall, 2006, Canadian Mortgage and House Corporation (CMHC) launched a demonstration initiative firstly named Net Zero Energy Healthy House, and later officially branded EQuilibrium Housing. CMHC describes this housing as follows (CMHC, 2006): EQuilibrium housing integrates high-performance, energy-efficient passive solar design and commercially available on-site renewable energy systems. They are designed to produce as much energy annually as they consume. Connected to the electricity grid, these homes draw power only as needed - and can feed excess power back into the system. Alouette Homes (AH) was selected as one of the twelve pilot demonstration teams. </p><p>In Canada, the total energy consumption in the residential sector was 1,296.1 petajoules (1015 joules) in 2005, which was about 17% of that years total national energy consumption (Statistics Canada, 2007). Residential passive solar technologies can greatly reduce the space heating fuel consumption (Athienitis and Santamouris 2002) and consequently the size of the mechanical equipment required. </p><p>Passive Solar Design </p><p>Incident solar radiation on the south-oriented facing faade and roof of a house generates a great amount of heat (Eicker, 2001). BIPV/T system using Photovoltaic (PV) arrays as solar thermal collector can harvest a considerable amount of this heat. This harvested heat together with the solar radiation transmitted through windows can decrease space heating load. In order to maximize the useful amount of collected heat and, at the same time avoid overheating, the excessive portion of this heat can be stored in thermal mass, such as concrete slabs for later use (e.g. at night) (Athienitis, 1997). </p><p>Warm/hot air flow from BIPV/T</p><p>Sun</p><p>Air intakes in soffit</p><p>Building integrated PV arrays</p><p>Air cavity</p><p> Figure 1. Configuration of houses BIPV/T system </p><p>Building Integrated Photovoltaics (BIPV) integrates of photovoltaics (PV) into the building envelope. The PV modules serve as the building outer skin. This integration can replace conventional building envelope materials. Overall cost is lowered. In BIPV/T system, the recovered heat can be collected and used for other </p></li><li><p>2nd Canadian Solar Buildings Conference Calgary, June 10 14, 2007 </p><p> 2</p><p>heating purposes (Charron et al., 2005). Figure 1 shows the conceptual design of AHs BIPV/T System. </p><p>Concrete is a common material used as thermal mass. For normal density concrete, its thermal conductivity (k) ranges from 1.2 to 2.0 W/(mk) depending on density and moisture content, and its specific heat is about 920 J/(kgK) (ACI, 2005). The solar absorbtivity and hemispherical emissivity of rough concrete are 0.6 and 0.91, respectively (ASHRAE, 2005). Concretes thermal characteristics provide the possibility for structural concrete to be used a thermal mass (Braham, 2000). However, to enable storage of heat in structural concrete, an integrated thermalstructural design must be followed. The concrete thermal mass thickness, exposed area, storage-restitution period and other parameters will affect its thermal energy storage density and thermal performance (Bilgen et al., 2002). Too much concrete mass will result in a heavy loading on structure; less than required concrete mass wont be able to accomplish thermal functions. In general, distributed thermal mass is preferable and a concrete thickness of about 20 cm has optimal thermal admittance (Athienitis and Santamouris, 2002). </p><p>MAJOR DESIGN CHARACTERISTICS The total floor area of AHs house is about 140 m2 excluding the basement floor area, which makes up another 90 m2. The house is designed to accommodate a four-person family. The house was designed to be energy efficient, and yet affordable and easy to construct. The houses energy performance reaches an EGH* rating of 98.3 (100 EGH* rating implies net-zero energy consumption, and 92 is the minimum required by CMHC). Fig. 14 is the schematic drawing of the houses service system </p><p>The overall energy analysis approach followed was a combination of HOT2000 (NRCan, 2005), RETScreen (NRCan 2007) and Mathcad (MEEI, 2001) custom software simulation. HOT2000 was used to optimize the envelope, Retscreen the PV electricity generation, and Mathcad software to optimize heat recovery from the PV and to avoid overheating due to solar gains i.e. optimize thermal mass in the house in relation to window area and determine the optimum window area. First, insulation was increased gradually while triple-glazed low-e-argon windows were quickly adopted as the most cost-effective measure to utilize passive solar gains incident on the south faade. Following rules of thumb (e.g. from Tap the Sun by CMHC) about aspect ratio of the house, a value of about 1.3 was chosen for this important variable. The design team then examined where effective thermal mass could be placed and significant mass was added in the direct gain zone (family room - floor and half wall). The </p><p>basement also contains a lot of exposed concrete mass that is utilized for storage of heat from the PV-thermal system. Based on Mathcad transient thermal simulations to determine the effectiveness of mass in storing passive solar gains while limiting temperature swings, a mass-glass combination was adopted resulting in a south-facing window area to floor area ratio of 9.1%. Increasing the insulation beyond the level adopted did not produce as much benefits as renewable energy measures. This was clearly shown at the design charrette through a combination of HOT2000 and Mathcad simulations. The high temperature swing of 5.5 C was selected in HOT2000 to increase the effectiveness of mass. </p><p>Envelope </p><p>The envelope of the house is well insulated. Windows are triple glazed with two low-e (0.35) hard coatings and 13 mm Argon filled gaps. The windows effective heat transfer resistance is 0.7 RSI. The total south facing window area of the ground floor is about 13 m2, which is approximately 15% of the ground floor area. The average effective RSI values for the walls above grade and roof are 6 and 8 RSI, respectively. The basement walls insulating value is 4 RSI; while the basement floor is 1.5 RSI. The wall thermal resistance values were selected following a sensitivity analysis with HOT2000. </p><p>Building-integrated photovoltaic-thermal (BIPV/T) System </p><p>The system was designed to cover one continuous south-facing roof surface. A 3 kW BIPV/T system is to be installed in the house. It consists of 22 Unisolar PVL-136 laminates attached to the metal roofing (each panel is rated at 136 Ws for a total of 22x136 = 2992 Ws). The electricity generated by the BIPV system as determined by RETScreen is 3420 kWh/yr for a 30o slope. A gap is created between the arrays and the sub-layer behind it. Outdoor air is used as the heat transfer fluid for simplicity and practical construction purposes. It is an open loop system so as to keep the temperature of the PV panels as low as possible, thus increasing their electricity production. This system is expected to produce up to 12 kW of heat at 500 cfm of air flow. Hot outlet air is to be used for domestic hot water heating, clothes drying, or thermal mass heating in order of priorities. </p><p>HVAC System </p><p>The primary heating and cooling equipment is to be a 2.2 ton two-stage water-to-air geothermal heat pump with an ECM (electronically commutated) motor. This heat pump uses well water, at 8.3 gpm (from 160 deep </p></li><li><p>2nd Canadian Solar Buildings Conference Calgary, June 10 14, 2007 </p><p> 3</p><p>well) which has been found at the site. The heat pump, through its desuperheater is also estimated to provide about 700 kWh/year of heating for domestic hot water . The ventilation rate is to be controlled by an occupancy sensor. When an occupant enters the house, the sensor will switch the heat recovery ventilator (HRV) to regular mode, which sets the ventilation rate at 0.3 ACH (air changes per hour). When the house is not occupied, the last person leaving the house can switch the ventilation rate to 0.15 ACH. This control strategy will reduce the heat loss due to ventilation, which is around 5070 kWh, as estimated by software HOT2000 (NRcan 2003) simulation. </p><p>Domestic hot water (DHW) heating </p><p>The system consists of two 60 gallon tanks in series. The entering cold well water is first heated by a drain water heat recovery system, which then enters the first storage tank. In the first storage tank, the water will be heated using the desuperheater of the geothermal heat pump and the hot air from BIPV/T system (except in winter time) through an air-water heat exchanger. The DHW will be heated up to the final supply temperature in the second storage tank using a backup electrical heater. </p><p>Thermal Mass </p><p>Exhaust outside</p><p>Passive Charge Thermal Mass (4" Concrete Slab)</p><p>Active Charge Thermal Mass (4" concrete slab, stores heat for hot air) </p><p>Warm/hot air flow from BIPV/T</p><p>Sun</p><p>Warm/hot air can be also used for hot water heating, cloth drying</p><p>Ground</p><p> Figure 2. BIPV/T plus thermal mass system. </p><p>Concrete slabs are to be used as the thermal mass. Figure 2 shows the placement of the concrete mass. One 100 mm thick concrete slab on the ground floor is used as direct gain thermal storage to store solar energy transmitted through the south-facing windows of the ground floor. Meanwhile, the concrete slab in the basement is used to actively store the heat from the hot/warm air heated by BIPV/T system while the release of the heat is passive. </p><p>For the basement slab, concrete is poured on steel decks which form air channels as shown in Fig. 3. Heat is stored in the concrete as hot/warm air from the BIPV/T passes through. Room air temperature is allowed to rise 5.5 oC from 21 oC (heating setpoint) to maximize passive solar heating. In order to have a higher relative friction factor (f) to generate larger convective heat transfer coefficient hc inside the channels, a layer of crushed gravel or metal mesh will be placed under the steel deck. </p><p>115</p><p>89</p><p>76</p><p>64</p><p>38</p><p>Normal Density Plain ConcreteSteel Deck (Canam P-2436, galvanized steel)Ventilation Channel (cavity)Metal Mesh (e &gt; 5mm)Rigid InsulationWater/vapor BarrierGravel (earth)</p><p>Unit in mm</p><p>Th_c</p><p>nc</p><p> Figure 3. Cross section of basement concrete slab </p><p>SIMULATION AND ANALYSIS A transient explicit finite difference thermal network model was developed to simulate the thermal performance for the entire house, including the BIPV/T and the ventilated concrete slab in the basement. </p><p>BIPV/T System Simulation </p><p>Air Flow</p><p>h.c_rf</p><p>h.c_gp</p><p>h.c_gp</p><p>T.o</p><p>T.pv</p><p></p><p>T.sf</p><p></p><p>S.pv</p><p>h.r_gp</p><p>U.ins</p><p></p><p></p><p>T.o</p><p>ExteriorPV ModuleAir GapRoofing MemberaneSheathingInsulationAttic Space</p><p></p><p>h.r_ps</p><p></p><p> Figure 4. Thermal network for BIPV/T system (one </p><p>section) </p></li><li><p>2nd Canadian Solar Buildings Conference Calgary, June 10 14, 2007 </p><p> 4</p><p>A quasi-two-dimensional finite difference model is used in the simulation part of BIPV/T system. Fig. 4 shows the thermal network of one section of the BIPV/T system. The BIPV/T system is divided into 30 cm sections along the direction of rise of the roof, one section after another, total 5.4 m long. The outlet air temperature of each section is calculated by solving (eq. 1) (Charron et al., 2005). The outlet air temperature of one section is used as the inlet air temperature for the section following the preceded section. The attic space temperature Tat is assumed to be 10 oC higher than outdoor temperature. A coupled attic model will be developed in the near future. </p><p>TairL x+</p><p>Tpv Tsf+</p><p>2TairL</p><p>Tsf Tpv+</p><p>2</p><p>e</p><p>x 2a</p><p>+ (1) </p><p>where c</p><p>airair</p><p>hWCM</p><p>a</p><p>= </p><p>The exterior convective heat transfer coefficient for heat transfer between PV surface and outdoor air is calculated as a function of wind speed (eq. 2) (Kreoth and Bohn, 2001). </p><p>Nu 0.036 Prair</p><p>13 Re</p><p>45 23200</p><p> Re 5 10</p><p>5&gt;if</p><p>0.664 Re</p><p>12</p><p> Pr</p><p>13</p><p> otherwise (2) </p><p>0 5 10 15 20 250</p><p>5</p><p>10</p><p>0</p><p>5</p><p>10</p><p>Convective Heat Transfer CoefficentWind Speed (mid-height of roof)Convective Heat Transfer CoefficentWind Speed (mid-height of roof)</p><p>Hour of the Day</p><p>W/(m</p><p>^2*K</p><p>)</p><p>m/s</p><p> Figure 5.BIPV/T exterior convective heat transfer </p><p>coefficient as a function of wind speed </p><p>The sky temperature for radiative heat transfer calculation is a function of outdoor dry bulb and dew point temperature (eq. 3) (Duffie and Beckman, 1980). </p><p>TskytimeTotime</p><p>0.8Tdptime</p><p>273</p><p>250+</p><p>0.25</p><p> (3) </p><p>Inside the BIPV/T cavity, when the air velocity ranges from 0.4 to 0.8 m/s, the air flow is in transitional range. </p><p>The entire flow path, 5.4 m long, is within the entrance region. The convective heat transfer coefficient for the heat transfer between passing air and the two surfaces (PV bottom and roof surface) is calculated using equations 4 to 6. A correlation (Kreith, 2001) for the average Nu number is employed. For air velocity at 0.5 m/s, the hc is 8.4 W/m2K. </p><p>Lentrance 0.034 Re Prair Dh (4) </p><p>Gz Re PrairDh</p><p>Hgap</p><p> (5) </p><p>Nu 3.657 0.0668 Gz</p><p>13</p><p> 0.04 Gz</p><p>23</p><p>+</p><p>1</p><p>+ (6) </p><p>Regulating the air velocity controls the the outlet air temperature. This is necessary for the purpose of having air at different temperature for different usages, and for maximizing the useful amount of heat collected. The flow velocity of the air under the PV will vary from a minimum of 0.4 m/s to a maximum of about 0.8 m/s. </p><p>0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.430</p><p>34.57</p><p>39.14</p><p>43.71</p><p>48.29</p><p>52.86</p><p>57.43</p><p>62BIPV/T Air Oultet (v=0.8m/s,To=30C,I=900W/m^2)</p><p>Distance from Inlet (m...</p></li></ul>


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