Solar Thermal - Types of system

Let’s start at the top…literally.

There are two types of collector: Flat Plate and Evacuated / Vacuum Tube.

Flat Plate Collectors

Flat Plate Collectors are less expensive, but more robust. They are less expensive quite simply because they do not perform as well as vacuum tube panels. However some people prefer the look of them. Flat plate collectors are made of a metal collector plate, usually copper, coated in special materials and fitted into an insulated box. A flat plat collector has an “absorber”, which is used to conduct heat through a metal sheet and is coated in a dark coloured “spectral-selective” paint” i.e. the paint is designed with a layered structure which optimises the conversion of short wave solar radiation into heat.

The average “annual” efficiency of a Flat Plate collector is 40%; this means that a 1 meter squared collector will generate around 400kWh of energy per annum.

In the UK each meter squared area receives around 1,000kWh of radiation per annum.

There are many advantages of Flat Plate Collectors:

  1. They are cheaper than Evacuated Tube Collectors.
  2. They can be a built into the roof – a fully integrated system.
  3. They are aesthetically more pleasing.
  4. They have a very good price to performance ratio.

Evacuated / Vacuum Tube Collectors

Evacuated Tube Collectors are more expensive than Flat Plate collectors but offer significantly superior efficiencies.

Very thin copper pipes carry “the fluid” within vacuum sealed solid glass tubes. The tubes act like a magnifying glass heating the fluid. The tubes are vacuum sealed in order to reduce thermal losses through air convection. The copper tubes can be 180 degrees hot and the tube glass will just be warm to touch.

There are many advantages of Evacuated Tube Collectors

  1. They are more efficient as there is no loss of energy in the tube
  2. They are less expensive than Flat Plate collectors
  3. They work better all year round
  4. They work better in low radiation
  5. They achieve high output temperatures
  6. Each tube can be replaced individually if they break

There are two types of evacuated tube collectors

Direct Flow-Through

This is a tube-in-tube system. The “absorber” (usually a copper tube) is heated and fed through to the base of the panel where it flows to your cylinder or thermal store unit.

Heat Pipe

In this system a “heat pipe” (usually a copper pipe) is inserted into the vacuum tube. The “heat pipe” is the “absorber” and is filled with an alcohol liquid. This liquid boils at an extremely temperature of 25 degrees Celsius. As it boils it turns into a vapour which rises up into a heat exchanger to transfer the heat. The vapour then turns into condensation as it cools and flows back down to the bottom of the heat pipe to start again. For this system to work efficiently the panels must be tilted to at least 25 degrees.

The average “annual” efficiency of an evacuated tube collector is 50% meaning that a 1 meter squared collector will generate around 500kWh of energy per annum

In the UK each meter squared area receives around 1,000kWh of radiation per annum.

Sizing your Collector

For Domestic Hot Water systems a simple rule of thumb to use is:

  • Flat Plate Collector – 1 to 1.5m squared of area per person
  • Evacuated Tube Collector – 0.75m to 1m squared of area per person

Industry standards suggest that each person in the house uses between an average of 60 litres of hot water per day at an average temperature of 45 degrees Celsius.

The Cylinder

In most cases of a solar thermal installation you will require a new cylinder for the reason that you will need a secondary coil.

A coil runs from your existing boiler through the cylinder where the heat from the coil exchanges with the water in the cylinder to heat it.

There are two coils in solar thermal cylinders; one from your solar thermal and one from your boiler.

Your mains cold water will enter the cylinder at the bottom of the cylinder and exit at the top.

The solar thermal coil is always at the bottom of your cylinder thereby heating your water first. As the hot water begins to rise, if it is not hot enough, a temperature sensor will “call” your existing boiler to start working to bring the water up to temperature.

Solar thermal can work with combi boilers but it is important to confirm with the manufacturer that it will accept pre-heated water.