Hot Water and Energy
Water heating typically constitutes between one quarter and one third of a household’s energy use and much more if using mains electricity. In most areas of Australia, between 65 and 80 per cent of domestic hot water demand can be supplied by a solar hot water system.
The concept of heating water using energy from the sun has been around for at least 100 years. The first patent for a solar water heater was awarded in Baltimore, Maryland, USA in 1891.
The components of a solar hot water system
All solar water heaters have three major components:
- Collector panels, which collect the sun’s heat and transfer it to the water
- Storage tank, which holds the heated water
- Boosting, which provides additional heat during cloudy days
Of these, the most important in relation to solar access are the collector panels – they gather heat from the sun. There are two main types of collector plates: flat-plate collectors, and evacuated tubes.
1. Flat-plate collectors
A flat-plate collector is a flat, insulated box containing an array of tubes connected to a metal absorbed plate, all painted black. The tubes and plate are fitted in a metal casing with insulation material behind a glass cover.
The glass is often low-iron tempered glass, which is quite strong. Low-iron glass is preferred because it reflects less of the incoming energy and retransmits less of the energy captured. Its performance is about 10 per cent better than plain glass.
2. Evacuated tube collectors
Evacuated tube collectors are becoming popular. They consist of an array of borosilicate glass tubes connected to a support frame, with a header tube across the top which joins all the evacuated tubes and acts as a heat collector. Inside each glass tube is a second tube and the space between the two tubes is a partial vacuum. The vacuum provides considerable insulation, so the inner tube, which has a heat-absorbing coating, heats up rapidly.
It transfers the heat to a copper tube inside it, which then transfers the heat to the water in the storage tank. Evacuated tube collectors can be up to 50 per cent more efficient than comparably sized conventional flat-plate collectors and perform better in overcast conditions, including providing frost protection.
Types of solar hot water systems
Solar water heaters are available in two main configurations, with models designed to suit sites with and without mains water connection. A solar hot water heater’s generally simple design allows it to be a useful and effective means to heat and pre-heat water for the home. The simplest and most reliable systems depend on the thermosiphon circulation of water (see right). When the sun shines on the collector panels, its heat is absorbed by the collector plate and conducted into the water in the pipes attached to that plate.
As the water rises in temperature it behaves like hot air and rises up the tubes to the top of the collector panel where it flows up through a header pipe and connecting pipes to a storage tank. Colder water from the bottom of the storage tank is then drawn down to the bottom of the collector panel to be in turn heated by the sun.
1. Close-coupled systems
These are the most common and easily recognised, and are usually mains pressure systems. The horizontal storage tank is fixed to the roof immediately above the collector panels. They require no pumps, and the tank can be located inside the roof to improve aesthetics.
2. Split systems
Pump-circulated or split systems can be used when it is impossible, or visually unacceptable, to have the tank above the collector panels on or in the roof. In this situation the water must be pumped around the system and down to the storage tank on the ground. This is also the case when retrofitting an existing mains pressure system.
This system has the advantage of having the element, thermostat and most valves at ground level for ease of maintenance and replacement. There is no concern about the weight of the tank on the roof, and the tank is a much better design for stratification.
Another advantage is that the tank can be installed first, operating as a conventional hot water service, and the solar collectors can be fitted at a later stage. One disadvantage of the system is that the pump draws power when operating.
What size hot water system do I need?
Determining the size of the solar water heater is the most important aspect of system selection. An undersized system will disappoint the user and require excessive energy boosting, thereby wasting non-renewable resources.
The system should be sized for the home and not necessarily its occupants as people sell houses and move on, but the hot water unit stays.
It is generally accepted that a person will use about 50 litres of hot water per day. In a storage system, it is good to have 1.5 days capacity. A three-bedroom house can comfortably accommodate four people and so it should have a 300-litre system, though it may be occupied by only two people.
The collectors are sized similarly. Panels are generally approximately 2 square metres each in size. You should allow 1 square metre per person. This means a 180-litre system will have one panel and is suitable for one or two people, a 300-litre system will have two panels and is suitable for three to four people and a 440-litre system will have three panels and is suitable for five to six people, or for a home with more than four bedrooms.
The best location
Solar hot water systems should be located where they will not have any shadows falling across them during the day. Shadows drastically reduce the ability of a panel or tube to heat water – and therefore the efficiency of the overall system.
- Feature image credit to ©iStock.com/nikkytok