Greenhouse - Solar Water Heating System

Introduction

The sun’s rays can be harnessed in a number of ways that are known collectively as solar power. The three main categories are:

Passive solar gain - maximising south-facing windows to capture the sun's heat.

Photo-voltaic solar panels - for generating electricity.

Solar water heating - solar collectors that transfer the sun's energy to heat water.

Home energy use accounts for around a quarter of all UK carbon dioxide (CO₂) emissions⁴. Household water heating accounts for over a fifth of the total energy used in the home³ (see figure 1). It therefore provides a significant opportunity to reduce your impact on the environment and make your home more sustainable.

Figure 1. Domestic energy use [Source: Boyle, 1996]³.

The Solar Resource

The amount of solar energy received at the Earth's surface decreases as you move north from the equator. In the UK, we receive 60% of the total solar radiation received at the Equator³.

Figure 2 (overleaf) shows how the average solar radiation received varies across the UK. The map shows how the solar resource decreases to the north. However, even in Scotland, one square metre of collectors can harness around 800 kWh* per year. The contours only represent solar radiation onto a flat surface so a south-facing angled surface will receive more energy than is shown.

Figure 2. The solar energy resource of the British Isles (kWh/m²/year). The map shows the solar radiation reaching a flat surface of one square metre. A solar collector placed on a south-facing angled surface will harness more energy than this [Information adapted from Hill, O’Keefe & Snape, 1995]⁵.

In winter, there is less sunlight so it is unlikely that solar energy can provide the total water heating requirements of a house in the UK. However, an efficient system can halve the annual cost of domestic water heating.

Water Heating Technology

Solar water heating is a domestically available and financially viable form of renewable energy. The time it takes to recover the initial cost is relatively short and it can if desired be fitted by a competent do-it-yourself enthusiast. Once operational, a solar system should supply up to 50% of your domestic hot water requirements, even in winter it can significantly reduce costs.

The Greenhouse, Norwich’s Environment Centre, has fitted Evacuated Tube Collectors (see figure 3), which the Trust considered the most effective for domestic use. These collectors show a rapid response to radiation levels, are lightweight and frost-resistant⁴.

The solar hot water system consists of a set of vacuum tubes to reduce heat loss, attached to a header pipe, through which water flows. Within each evacuated tube is an absorber strip with a semi-conductor layer that absorbs solar radiation and the energy is passed through a ‘heat pipe’ to the water circulating through the header pipe at the top³.

The heat pipe is filled with liquid at a controlled pressure to allow boiling and condensing within the tube. This enables large energy transfers for small changes in temperature³. Water running through the header pipe travels to a heat exchanger coil in the hot water cylinder where it heats the mains water supply.

Figure 3. An evacuated tube collector [Redrawn from Boyle, 1996]³.

When the temperature difference between the water running through the header pipe (on the roof) and that in the hot water cylinder is greater than 5°c the pump is automatically switched on to activate the system.

The system operates whenever solar radiation is available but the sun does not need to be shining in order for the system to collect solar energy. In the UK, over half of the available solar energy is diffuse radiation (re-radiated from clouds and the atmosphere). The remaining energy is harnessed from direct sunlight⁶.

The Greenhouse system uses Thermomax Evacuated Tube Solar Collectors. These provide high performance in all weather types, an advantage over the alternative flat-plate collectors⁷. Thermomax recommend that 30 collector tubes should suffice for a family of four. The Greenhouse requires a larger area to supply hot water for the caretaker’s flat and the café kitchen that caters for up to 80 people, including the necessary washrooms.

* The standard numerical unit of energy is the watt, a measure of the rate at which energy is used or supplied. An electric kettle consumes electricity at a rate of about 1000 watts in one hour, or one kilowatt-hour (kWh). Equally, one bar of an electric fire gives out one kWh of heat in one hour.

*An Evaluation of the Greenhouse Solar Hot Water System.*	Previous	Contents	Next
1. SOLAR WATER HEATING