Energy

Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Photovoltaic systems

Photovoltaic systems (PV or solar electric systems) absorb sunlight and convert it into electricity. They can be used as part of a stand-alone power system in remote locations, or as partial replacement for mains supply.

Find you about:

  • advantages and disadvantages
  • configuration
  • capacity
  • maximising sunlight absorption
  • types of solar cell
  • efficiency
  • array frames
  • consents and permits
  • insurance
  • costs
  • warranties
  • photovoltaic system location.

Advantages and disadvantages

Advantages of photovoltaic systems are that they:

  • are quiet
  • are non-polluting
  • have low operating costs
  • have low maintenance costs
  • contribute to reduction of greenhouse gas emissions.

Disadvantages of photovoltaic systems are that they:

  • have high initial capital costs
  • require batteries if an off-grid installation
  • may not meet all the household electricity needs.

Configuration

A photovoltaic array is made up of solar PV panels that contain solar cells. The cells consist of layers of semi-conductor material (typically silicon), generally sandwiched between glass and another robust material and are sealed against moisture.

Solar radiation striking the cells cause electrons to move between the semi-conductor layers, creating an electric current. Cells are connected to produce a voltage output from the panel.

Capacity

The electricity generation capacity of photovoltaic panels is measured in peak Watts (Wp), which is the panel’s peak power output rating under standard test conditions.

Panels come in output capacity sizes ranging from 5–200 Wp and can be configured in any array size. An array of panels with a 2,000 Wp rating may produce between 4 kWh and 10 kWh per day on sunny days with good solar gain (New Zealand households use an average of 22 kWh of electricity per day). Residential installations range from 500 W to 10 kWp with most between 1–3 kWp of output.

Maximising sunlight absorption

The capacity of any given photovoltaic system is directly proportional to the amount of sunlight absorbed, which depends on these factors:

  • Solar irradiance – This is generally higher at more northern latitudes, in summer, in clearer air and when there is less shading. Avoid shading – shade on even a single cell can disproportionately affect the power output of a panel. Photovoltaic cells can still generate electricity in cloudy conditions, though at a lower output.
  • Solar panel area – Approximately 1 kW maximum or peak electrical power can be generated under standard conditions from between 5–17 m2 of solar panel, depending on type.
  • Solar panel orientation – In New Zealand, the sun is always in the north for all locations and all times of the year. Therefore the solar panels should, in general, be oriented to the north– for maximum solar irradiance, they should be oriented directly north. However, it may be appropriate to change the orientation a little to coincide with higher electricity demand – for example, turning the panels slightly to the east if demand is higher in the morning, or slightly to the west if demand is higher in the afternoon. It may also be necessary to change the orientation because of shading, aesthetic reasons, lack of available space or poor building orientation. As general guidance, it is not recommended that the panel be orientated more than 20 degrees from true north.
  • Solar panel tilt angle – The tilt angle is the angle of the solar panels to the ground. For a grid-connected system that aims to generate the maximum amount of energy on an annual basis, the tilt angle should be at the local latitude. Off-grid systems are usually designed to maximise output in winter when power need is greatest, so tilt angle should be local latitude plus 10º. Systems that allow the tilt angle to be adjusted maximise efficiency.
Tilt angle 
Tilt angle

The tilt angle is the angle of the photovoltaic panels to the ground. The ideal angle will be different in summer and winter, and depends particularly on how the system is used.

Types of solar cell

There are two common types of solar cell panel:

  • Crystalline silicon solar cells have a solid silicon wafer as the semi-conductor. The cells are sandwiched between tempered glass and a backing of tough ethylene vinyl acetate (EVA). These cells are protected from moisture. They need to remain cool as their output efficiency can drop by about 0.5% for every degree Celsius above a standard test temperature of 25ºC. They typically incorporate a gap of approximately 150 mm behind the panels to allow for cooling.
  • Amorphous silicon thin film solar cells have silicon in a thin film as the semi-conductor. The silicon thin film is deposited on a low-cost substrate such as glass or a thin metal foil. The coating on top may be a flexible material (as opposed to glass), and they may use a flexible mounting system. This type of cell is generally cheaper. They are being developed for integration with materials so they can be part of the building fabric.

Efficiency

The average efficiency of a well-located photovoltaic panel ranges from 5–20%. Efficiency varies depending on the type of cell used: multi and mono crystalline cells are generally between 12–16% efficient; amorphous silicon and thin film PV are around 5–9% efficient.

Efficiency gives an indication of the amount of space required and is not necessarily an overall measure of a system’s advantages. Some lower efficiency technologies such as amorphous silicon can have a high energy yield per kWh.

Array frames

Array frames allow the solar panels to be tilted to the optimum angle for receiving solar energy. They can be:

  • fixed (permanently oriented in one direction)
  • adjustable (so the orientation can be changed to suit the time of year)
  • tracking (which move to follow the sun).

Tracking array frames are normally controlled by an electric motor or a refrigerant gas. They provide more electrical power output throughout every day of the year (there may have some power used to provide the tracking, but this will normally be less than the additional power output obtained). They are likely to be more beneficial in southern locations. They are more expensive than the alternatives, require more maintenance and may be less reliable. Therefore, it may be more economical to use more array frames to increase power output than to employ tracking frames.

The array frame must be installed to ensure it:

  • meets wind and seismic loading – some frames designed for European and North American situations may not suit New Zealand’s coastal and windy environments
  • is isolated to prevent electrochemical corrosion with different metals in the solar panels or the building fabric – New Zealand metal roof manufacturers specify a 100 mm gap so that panel installations allow for roof washing and do not void roof warranties
  • allows adequate airflow behind the panels to provide cooling – approximately a 150 mm gap for crystalline silicon panels.

Consents and permits

Building or resource consents may be required for photovoltaic systems that penetrate the roof or are considered by neighbours to affect their property.

Any grid-connected photovoltaic systems need to be agreed to by both the lines company (for the connection) and the electricity retailer (for pricing arrangements).

The design and installation of the system should be carried out by skilled tradespeople to ensure safety and energy-efficient outcomes, and work must meet the requirements of AS/NZS 3000.

Insurance

Insurance coverage is as for all other electrical equipment, although the building owner may want to check the limits of coverage with their insurer.

Costs

A photovoltaic installation is currently very expensive to buy and install, although prices will fall as global demand and production grow, and more cost-effective alternatives are developed.

In 2004, the costs of photovoltaic systems were up to NZ$17 per watt installed, which meant that fitting a 2 kWp system cost almost $40,000. In 2009, costs are more likely to be between $9–$13 per watt installed, making a 2 kWh system cost between $18,000 and $26,000.

Those costs will include cabling, metering apparatus, mountings and frame for panels, and any consents required.

In remote locations, the cost of connection to the electricity distribution grid (which can be between $20,000 and $25,000 per kilometre) makes photovoltaic systems more immediately economic.

Warranties

System warranties are likely to be for 5 years, including a workmanship warranty on the panels plus a 25-year limited warranty of power supply, and a 5–10-year warranty on the inverter.

More information