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 We expect to get up to 90% of the hotwater heating demand from the solar water heating system

Solar Water Heating (SWH)

Many of the design features including the building envelope and systems help to minimise the energy demand of the house.  We expect these features will allow us to use between a third and a half the energy of an average New Zealand house.

Water heating in a typical New Zealand home typically consumes 30%1 of the energy used by the building. An additional 30%1 is consumed by space heating. As the design of the ZEH avoids the need for space heating we expect hot water usage to be between 40% and 45% of the energy usage. For a typical home this equates to 3200kWh or ~$765/year.

The use of low flow shower heads and other low flow fittings will reduce the hotwater demand. Some shower fittings use over 20l/s, it is possible to still have a good shower using on 10l/s.

Solar hot water performance

The chart below shows expected energy usage over the year. The yellow area is the maximum expected to be generated by the solar water heating system, the remaining orange will be produced by the backup heating element. We expect to get up to 90% of the hotwater heating demand from the solar water heating system, a 35%-40% saving in energy usage for the house. This is higher than typically expected and is a result of the system being sized larger than usual.

Solar water heating percentage contribution to annual hot water demand

The solar water heating system

Basic solar water heating system schematicWe plan to install a solar water heating system from SolarCity1, utilising Thermocell hot water collectors. These are flat plate collectors that utilise a heat pipe technology to transfer heat up the plate where it is collected by water, pumped from the tank. The basic schematic is shown to the left.

Thermocell hot water collectors have been manufactured in Christchurch for 30 years and have proven to be reliable and well performing over this time. We are excited to be able to use a locally made innovated product.

 

The solar hot water collectors will be installed along the top of the north facing roof, in the same plane as the solar PV modules.

Proposed solar array layout as of 25/05/2010

Solar hot water technologies

There are two main types of solar hot water collector used for domestic hot water heat. Both operate using the same basic concept:
  1. a collector plate is heated by the suns energy
  2. to minimise the amount of this heat being lost back to the air the collector plate is insulted
There are three main factors influence the efficiency of the collector:
  1. How much solar energy falls of the collector plate (primarily a function of the type of glass, the location and therefore available solar energy, and the angle/direction the collector is installed at)
  2. How much of the solar energy that falls of the collector plate is absorbed and transfered to the water (and not re-radiated)  (primarily a function of the solar collector plate coating)
  3. How much of the energy absorbed by the collector plate is lost back to the air (primarily a function of the insulation)
The two main types of collector designs are:
  1. Evacuated tubes - the solar hot water collector plate is insulated by an evacuated glass tube
  2. Flat plate - the solar hot water collector plate is insulated by a minerial wool (or similar) and the front by a sheet of glass
Flat Plate solar hot water collector array Evacuated tube solar hot water collector
The technologies while quite different in appearance both function with similar efficiency at typical hot water temperatures. There is a perception that evacuated tubes are significantly more efficient than flat plate collectors. This is true at high temperatures, say above 65°C, but does not significanlty improve the solar contirbution that can be obtained for a home as most homes require hot water at less than 60°C.

Meeting the project goals:

Energy Efficiency and Green House Gas: 
  • Energy generated by solar water heating has zero operating CO2 emissions per kWh compared to electricity from the grid which is ~180g/kWh on average in New Zealand. 
  • The Thermocell panels that we are planning to use have approximately a 6 month energy payback period.

Footnotes:

  1.  BRANZ HEEP Study
  2. Shay works as the Head of Design and Innovation at SolarCity