Energy

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

Solar Water Heating: Collector Panels

The two types of solar panel commonly available are a flat plate collector or an evacuated glass tube collector. Also available are panels incorporating a heat absorbing surface that transfer the heat to water pipes below.

The size of the panel depends on the water storage capacity available and is typically based on a ratio of about one square metre of panel for each 40-70 litres of cylinder volume. Most panels commercially available are in the 3 to 8 m² range.

For a home with 3-4 occupants, a 200-275 litre storage cylinder with 2.5-5 m² panel is recommended. For 5-6 occupants, the recommendation is for a 270-360 litre cylinder with a 3.75-6 m² panel area.

  • The expected output from a solar panel installed to maximise the amount of sun it receives is up to:
  • 15 kWh/day on a sunny summer day, and
  • 7 kWh/day on a sunny winter day (typically water heating requires an energy input of 6-10 kWh/day).

Flat plate solar panels

These are the most commonly used collectors and have a metal plate (often copper though sometimes aluminium) to absorb heat.

The plate has a dark-coloured coating. Pipes under the plate absorb heat and carry it to the storage cylinder. A transparent cover (normally glass) lets in solar radiation but stops it from escaping.

Flat plate panels have a life expectancy of about 20 years. Their construction is simple, with no moving parts, which means they’re easily repaired. They are relatively low cost, and can supply water at temperatures up to 95ºC, though efficiency diminishes rapidly above 70ºC.

Evacuated glass tube solar panels

These are made up of a number of glass tubes (normally 20 or more). Each tube has a vacuum to reduce convection and conduction heat losses. Inside the vacuum a plate (often made of copper) absorbs heat. A tube, cylinder or pipe connected to the absorber plate contains water or some other fluid such as glycol which absorbs heat and carries it to the water in the storage cylinder.

Evacuated glass tube solar panels 
Evacuated glass tube solar panels

These are made up of a number of glass tubes (normally 20 or more). Each tube has a vacuum to reduce convection and conduction heat losses. Inside the vacuum a plate (often made of copper) absorbs heat. A tube, cylinder or pipe connected to the absorber plate contains water or some other fluid such as glycol which absorbs heat and carries it to the water in the storage cylinder.

 

 

 

Evacuated glass cylinders are more efficient than other panels when the sun is striking at an angle lower than 90º, when there is less insulation, and when ambient temperature is lower. Individual tubes sometimes fail but can be replaced at relatively low cost.

Solar heat absorption

The heating capacity of solar water heating systems is directly proportional to the amount of solar radiation absorbed, which depends on:

  • Solar intensity – The available solar generation capacity is normally determined by the available solar radiation energy (kWh/m²) for each location. Solar intensity is generally higher at more northern latitudes, in summer, in clearer air, and when there is less shading.
  • Solar panel area – The larger the area, the more heat can be produced.
  • Solar panel tilt angle – The tilt angle is the angle of the solar panels to the ground. The ideal installed angle for solar absorption is perpendicular to the sun. Over the year this is equal to the latitude (for example, 41.2º in Wellington), however winter sun angles are lower than this and summer sun angles are higher. If winter water heating loads are especially high, the panels can be optimised for winter by making the angle 15º more than latitude.
  • 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 intensity, they should be oriented directly to geographic or solar north.

It may be necessary to change the location, tilt angle or orientation because of shading, aesthetic reasons, lack of available space, complex roof profile, or lack of structural support from the building. However, the orientation should never be more than 20º from north, as solar absorption will reduce by more than 20% (reductions in heat gain continue the further the panel is from the geographic north point).

Solar panel tilt angle 
Solar panel tilt angle

The tilt angle is the angle of the solar panels to the ground. The ideal angle will be different in summer and winter.

For your clients

To help your clients make decisions about water heating options, and to help them find ways of reducing their hot water bills, refer them to www.smarterhomes.org.nz/energy/water-heating and www.smarterhomes.org.nz/energy/solar-water-heating.