Position modules where they are never shaded at any time in the year; •
Position them at the correct angle to the sun for your latitude (this is different •
for grid-connect and off-grid systems);
Face them towards the equator (usually, these can be exceptions to this); •
Position them where they can be reached easily for cleaning and maintenance; •
Use a mount made of durable, corrosion-free materials; •
High temperatures reduce the efficiency of the modules. Therefore allow •
appropriate ventilation behind the modules to dissipate heat.
Table 6.3 The effect of orientation and tilt on PV electricity-generating potential. Orientation Tilt Generation (kWh/m2/day) Difference from optimum (%)
South 35° 3.00 optimum South vertical 2.18 17 South horizontal 2.63 12 East/West 35° 2.46 18 East/West vertical 1.63 47 East/West horizontal 2.63 12
Note: The output of a PV array in this location (an apartment block in Manchester, England, with no shading) is reduced by 17% when placed vertically as a building facade, according to these simulation results from the RETscreen software (see Chapter 10 Resources). If they are also east–west facing, then nearly half of the potential power is lost.
Figure 6.17 Optimum tilt angles for grid-connected
PV systems in the US, from US DoE Best Practice Guide for Solar Thermal & Photovoltaic Systems. Useful online software for angle selection is PVWatts for North America and PVGIS for Europe (see Resources). It is assumed here that the PV array is facing due south. Angles for off-grid and hybrid systems will be different.
Figure 6.18 How temperature
affects the performance of a PV cell. This cell is receiving 1000W/m2 in each case (the
three curves on the graph), but the higher the temperature, the sooner the voltage drops off and the less power it produces. 0 0 2 4 6 8 10 irradiance = 1.000 W/m2 75°C 50°C 25°C 12 5 10 15
Voltage Across Panel (V)
Panel Efficiency (%)
20 25
AM = 1.5
crystalline module will produce 6 per cent less power than under STC. The effect is less pronounced for thin-film modules, for which it would be 2 per cent less. Much effort goes into trying to prevent modules becoming too hot in hot weather/climates. One important factor is the movement of air behind the modules; if this is restricted, modules can overheat, so it is important to ensure ventilation behind them. If they are mounted on a rack, the air can move freely behind. If the modules are building-integrated, and are completely built into the structure of the wall or roof of a building, they must be ventilated behind. If the system is in a very windy area, this will reduce the temperature of the modules, which will help to slightly increase their efficiency.
Solar electric-thermal collectors
An attempt has been made to tackle this problem with a new, hybrid solar collector that incorporates both PV power generation and SWH (thermal) – called PV/T. The modules have other potential advantages. The IEA has concluded that PV/T can generate more energy per unit of roof area than the equivalent side- by-side PV modules and solar thermal collectors at a potentially lower production and installation cost. It is claimed that this is because the solar thermal inlet cools the PV component to increase their efficiency by up to 20 per cent, at 45°C (113°F), compared to an identical PV system alone. Various types of commercial products are available, including liquid and air collectors for the thermal component. One solution, for example, is a solar wall with PV cells on top. Another is a panel containing a solar air collector with smaller PV cells driving a ventilator, which sends fresh hot air into the residence. Tracking modules are Figure 6.19 A cross-section through a liquid-
based flat-plate PV/T module. The PV cells sit above the thermal collector, which helps to cool the cells.
Figure 6.20 The John Molson School of Business (JMSB) is a 24.5kWp building-integrated photovoltaic/thermal
(BIPV/T) installation in Montreal, Canada, that uses a high-efficiency distributed air inlet method to recover heat from the PV panels and feed up to about 424,650 litres per minute (15,000ft3 per minute) of pre-heated air into the building’s
HVAC system, to offset some of the fresh air heating requirements and cool the panels. The installers say it will yield up to approximately 100MWh of heating per year. The efficiency of the 288m2 of PV panels can be increased by this
cooling, by up to 5% on cold sunny days, compared to an installation without the cooling effect.
The IEA’s PV/T task group regards it as a very promising technology, with large potential for being applied in a significant proportion of the current solar thermal market, including domestic hot water systems. In the short term, multi- household buildings may be an important market, due to the limited roof area available per household. In the medium and long term, the most promising application seems to be domestic water heating and space heating. This is especially the case with advanced low-energy houses aiming to cover a large part of their energy needs with solar. The combination of a heat pump and PV/T could also be promising. In the longer term, the IEA sees more commercial applications in industry, commerce and agriculture and applications such as solar cooling.
For this to happen, greater standardization is required for performance and reliability, and financing schemes specifically targeted at this hybrid technology. Installation costs need to be reduced together with procedures for PV/T to become an integral part of building design. To this end, the IEA has developed a number of demonstration installations. This is as yet a niche market, however, it is theoretically an attractive concept – although hard to achieve technically – throughout all of the seasons. It would need to be installed by someone competent to meet the electrical and plumbing/heating standards in a given country. Financial support for solar PV
Usually, different tariffs apply for electricity sold to and bought from the utility company. Some countries have introduced ‘feed-in’ tariffs (FITs) that pay the
Figure 6.22 Electric-thermal joint collector with the
parabolic trough providing electricity.
Source: © IEA Figure 6.21 The PVTwin PV/T collector; pipes behind the
PV module heat water for use, while cooling the module to improve its performance.
Figure 6.23 Roof-mounted grid-connected/utility-interactive PV system on the Renewables Academy (RENAC) in Berlin,
Germany. The system consists of 3 PV arrays of approximately 1 kWp each. (a) Solon Blue 230/7 200 Wp polycrystalline silicon modules; (b) Sunset TWIN 140 140 Wp multi-junction thin-film silicon modules; (c) Inventux X115 115 Wp micromorph (a-Si/μc-Si) modules; (d) 3 SMA Sunny Boy inverter SMA 1100 inverters, one on each phase. Telecom line for system monitoring goes to the RENAC offices. Three different PV array types were chosen for training purposes. More information is available at www.renac.de.
Source: (a–c) Frank Jackson; (d) Alberto Gallego
supplier well over the going rate for solar electricity that is ‘fed into’ the national grid. This is a type of subsidy that can act as an incentive to install a renewable energy system, which will therefore pay for itself more quickly. Some governments, such as the UK’s, even give this tariff for all electricity generated, regardless of whether it is exported into the system or not. Those countries
that have introduced such a system, such as Germany, have seen a vibrant growth in the number of domestic installations with a knock-on effect on jobs in the sector. However, if the tariff is withdrawn suddenly, as happened in Spain in 2009, the industry will contract. FITs are financed either from general tax revenues or from an increase in general average electricity bills. Questions have been raised over the social equity of this policy.