BIPV SYSTEMS AND SOLAR SKINS Kamil Staněk CTU in Prague, Faculty of Civil Engineering, Department of Building Structure Thákurova 7, 169 29 Prague 6 e-mail:
[email protected]
Abstract Building Integrated Photovoltaics (BIPV) is rapidly developing concept of integration of hi-tech renewable energy systems into building envelopes. BIPV systems can be designed in form of façades, roofs, semitransparent PV glazing, etc. with emphasis on full functional, structural and aesthetical integration and cooperation with building. On going research project at Faculty of Civil Engineering, CTU in Prague is aimed at ventilated BIPV façades. As a part of the project, the 150 m2 experimental PV installation will be raised on southwest façade of the faculty with complex measurement of key quantities. At the same time simulation model is being developed to calculate and predict optimal construction and technological parameters of ventilated BIPV façades. Advanced concept of BIPV façade is solar skin which unifies advantages of PV systems and double-skin façades into architecturally attractive power generating entity.
2. Experiment On-going research project at the Faculty of Civil Engineering, CTU in Prague is aimed at ventilated BIPV façades. Large-scale experimental PV installation is under construction on the south-west façade of the faculty. Mechanically ventilated PV system is designed in two parts along a central glassed-in strip of the existing façade. PV panels will be mounted on a steel/aluminium supporting structure covering total area of 150 m2. Number of modules Type of modules Number of PV arrays
176 c-Si 105 Wp 3
Total area of PV arrays 150 m2 Total output power 18.5 kWp Number of sensors 50
Table 1: Basic parameters of experimental PV façade
Key words: BIPV systems, ventilated façade, simulation, solar skin
1. Introduction In general, photovoltaic devices convert energy of sunlight into electricity. This is happening instantly when photons hit a solar cell – the basic element of PV devices. Efficiency of such a conversion is about 15% (crystalline silicon based PV devices). Building Integrated Photovoltaics (BIPV) is rapidly developing concept of integration of PV devices into building envelopes. BIPV systems can be designed in form of façades, roofs or PV glazing with accent put on full functional, structural and aesthetical integration and cooperation with the building. During operation of PV systems neither noise nor pollution is produced. Advanced concept of BIPV systems is solar skin which unifies both opaque and semitransparent PV elements together with double-skin façades and ventilated roof systems into architecturally attractive energy generating building envelope.
Fig. 2: Visualization
Fig. 1: Ventilated PV façade scheme
Number of sensors will be installed on the façade. Temperatures will be measured on the back side of PV panels, in the ventilated air gap and on the wall surface. Air speed will be measured for both parts and incident solar radiation will be measured in two vertical levels – at the top and bottom of the PV façade. Mechanical ventilators equipped with speed controllers will be mounted at the top of the ventilated air gap and they will operate automatically according to temperature differences between the air gap and ambient environment.
3. Simulation For prediction of PV system behaviour and optimization of its structural, geometrical and technological parameters, a complex simulation model must be established. It basically consists of individual models which are solved simultaneously.
absorbed by the solar cells is directly converted into electricity and the remaining energy is turned into heat.
Fig. 3: Complex model scheme 3.1 Meteorological and airflow model The overall simulation model takes meteorological data and airspeed as input variables for computation of thermal and electrical energy output. Calculations of meteorological inputs are based on METEONORM data for Prague. Values of incident solar radiation on horizontal plane are recalculated for south-west orientation and 90° tilted façade surface [2]. Furthermore the values of ambient temperature and wind speed are needed.
Fig. 4: Network of solar energy conversion Thermal energy generated at the solar cells is conducted simultaneously towards front and back surface of the PV modules. This energy is radiated and convected from the modules’ surfaces to surroundings or to the ventilated air gap, respectively. The thermal model is then based on solution of 1D heat equation with mixed boundary conditions for conduction within the PV panels and for forced convection in the air gap [5, 6].
From the nature of airflow in the mechanically ventilated air gap follows that the velocity of moving air can be determined independently of the air temperature. Airspeed in the ventilated gap is obtained by analytical solution of Navier-Stokes equation and continuity equation for 1D stationary turbulent flow, where the air is considered to be newtonian uncompressible fluid [3]. 3.2 Electrical model Calculation of electrical energy output is based on Photovoltaic Array Performance Model [4]. The amount of output power depends, beside the incident solar radiation, on solar cells operating temperature – with increasing cells temperature, the efficiency of energy conversion is decreasing. That is why it is desirable to keep the cells’ temperature low, which can be done by mechanical ventilation of the air gap. For determination of a benefit of the mechanical ventilation an appropriate thermal model must be built. 3.3 Thermal model Incident solar radiation on the PV modules is either reflected or absorbed by a glass cover and the rest is transmitted to the solar cells. A portion of energy
Fig. 5: Network of heat exchange with key nodal temperatures Important is that thermal energy from the air gap can be collected and utilized for heating or cooling of a building. For instance during heating season the preheated air can be directed into ventilation unit and during summer time this hot air can drive desiccant cooling system.
4. Preliminary results and conclusion Preliminary calculations of PV façade behaviour are based on meteorological and electrical model, while airflow and thermal models are not incorporated yet. Figures below show computed values of key parameters for May.
Fig. 7 shows that a difference between PV panels’ backside temperature and corresponding ambient temperature is more than 15°C (in May). It follows that mechanical ventilation can significantly contribute to lowering PV modules’ temperature and increasing their efficiency. PV array output power
Global irradiation on titlted surface (vertical,SW) Total energy output 1825 kWh
Total energy input 108.105 kWh/m 2
12 output power [kW]
2
global radiation [W/m ]
800
600
400
200
8 6 4 2 0
0
149
149
145
145 141 137 133 129 125 121
day of year
1
3
5
7
9
141
25 21 23 17 19 15 11 13
137 133 129 125 day of year
hour of day
1
3
5
7
9
hour of day
The calculated values will be evaluated against measured data. Verified model can then be used for designing of various ventilated PV systems.
PV panels temperatures - day maxima 45 solar cells back surface corresp. ambient
40
121
25 21 23 17 19 15 11 13
Fig. 8: Total output power of the PV façade
Fig. 6: Incident solar radiation
References [1] The German Solar Energy Society: Planning and Installing Photovoltaic Systems – A Guide for Installers, Architects and Engineers, James&James, London, 2005
35 30 temperature [°C]
10
[2] Heinemann, D.: Energy Meteorology – Lecture notes, Carl von Ossietzky Universität, Oldenburg, 2002
25 20
[3] Šesták, J., Rieger, F.: Přenos hybnosti, tepla a hmoty, ČVUT v Praze, 2004
15
[4] King, D. L., Boyson, W. E., Kratochvil, J. A.: Photovoltaic Array Performance Model, Sandia National Laboratories, Albuquerque, 2004
10
[5] Incropera, F. P., DeWitt, D. P.: Introduction to Heat Transfer, 3rd ed., John Wiley & Sons, New York, 1996
5 0
121
125
129
133
137 141 day of year
145
Fig. 7: PV panels’ temperatures
149
[6] Mei, L., Infield, D., Eicker, U., Fux, V.: Thermal Modelling of a Building with an Integrated Ventilated PV Façade, Energy and Buildings 35 (2003) pp. 605-617
