Solar Hot Water Heating, why do it?
With the increased focus on being more energy efficient, there has been a lot of investigation into ways to reduce power consumption (and hence greenhouse gas emissions
) when producing hot water.
The 'old ways' of producing hot water either involved heating via an electric element, or via some form of heat exchange with another in house heat source - i.e. a gas or coal fire. With the increased cost of electricity and the very 'dirty' aspect associated with coal these are becoming no longer acceptable ways to provide your main means of heating water.
Solar hot water heating systems, how they work
Simply put a solar collector it put outside in such a way to maximize its exposure to the Sun. This solar collector is designed to convert the solar energy
into a direct temperature rise within the collector. This heat is then exchanged with the water flowing through it. This water is part of a, usually, pump driven loop back to the main water tank
. Where the incoming hot water rises the temperature of the water in the tank to make it all 'hot' and you end up with a tank full of hot water.
All the systems and solutions you will find are just variations on this simple design. Below we shall look at the differences in detail.
Gravity fed or pump driven solar hot water systems
There are two options you will often see: gravity fed or pump driven.
The gravity fed system has the main hot water tank at a slightly higher level to the solar collector. Using the fact that hot water will 'rise' in a cold water column, the top of the water tank is hooked up to the outlet at the top of the solar collector and the bottom of the water tank is hooked up the inlet at the bottom of the solar collector. This means when the water is heated in the solar collector it rises to collect at the top of the water tank, forcing the colder water at the bottom to run into the solar collector to be heated and the cycle continues..
The pump driven option allows the hot water tank to be positioned below the collector and through a controlling circuit the cold water is pumped into the solar collector.
The gravity system has the advantage of simplicity and no dependence on electricity to operate. Its downside is that you are limited as to exactly where you can place the water tank inside in relation to the solar collector outside. A variation on this is where the water tank and solar collector are in one unit; which is quite common - although the downside with this is the whole system is exposed to the elements and you cannot have a 'boost' element in it (to heat your hot water on cloudy days).
On the other hand, the pumped system places no real restriction on where the water tank is in relation to the collector. Although the further the water tank is away the more 'heat loss' will occur around the pipes; so insulation on the pipes is very important to have a well working system.
Types of solar collector used for heating water
These are grouped into two main classes; what we term the 'direct heat' class and the 'indirect heat' class.
The direct heat types basically heat the water directly within themselves - sort of like a radiator but in reverse, often term flat panel water heaters. These are simple to operate but suffer from the fact they cannot concentrate their heating to raise the water temperature dramatically over a short time. i.e. great for heating lots of water a little. This is often the system used for heating swimming pools with big areas of solar collectors. If they are used for heating house hot water, often the hot water tank needs to be placed very close (if not with) the panel, so in this way the water can quickly circulate and keep being heated.
The indirect heat types instead use the solar energy focused on some material - which gets very hot as result. This hot material is then usually introduced into the water flow to exchange its heat. So in this way slow flowing water can be exposed to a large amount of heat and so be quickly raised in temperature. One of the common instances of this involve using a copper rod enclosed in an evacuated (i.e. vacuum) tube with the end of the copper rod in contact with the water to be heated. Due to the vacuum in the tube the heat caught in the copper rod has nowhere to go but into the water. These are often called an evacuated tube solar hot water system.
The evacuated tube systems are great at quickly heating up a smaller volume of water due to their efficiency they often work reasonably well on most days (presuming not completely overcast). They also look better on the roof, as its a 'tidy' unit. Although for this you will pay more in initial costs, so it pays to shop around as there a number of suppliers of these systems on the market now (basically the evacuated tubes are standard component mass manufactured, probably in China, the variation comes in what these get mounted into).
Important things to remember with solar hot water systems
- Do make sure that the place where you want to mount your solar collector has good all year round Sun access, i.e. no trees or other buildings blocking the sun.
- Do make sure you have an additional means of heating your hot water. An electrical element in the water tank is the best way to go, but do make sure you have a conveniently sited on/off switch for this in the main house with a light indicating when its on - so its easy to see when its on and turn it off when not needed (this on its will save you a fortune!)
- Check that the hot water tank and immediately connected pipes (i.e. up to about a metre out) are well lagged and insulated. A lot of heat is 'lost' through the external fixtures and fittings to the tank as they cool and exchange heat with the external environment.
- If you have a pump driven system, really go to town on lagging the pipe circuit between the solar collector and the water tank - this will make sure every bit of heat captured is transferred into your hot water tank.
- Also do lag at least the first 2 to 3 meters of pipe leading out of the hot water tank onto the pipes, copper is very good heat conductor and lot of heat will 'leak' out down the pipe, as it is at the top of the tank and will act like radiator if not lagged!
- Also if you have the spare insulation, its also considered good form to lag all the way to the first junction off the top pipe, this is usually where it will go to a smaller gauge. So in effect you are lagging the larger pipe to create a 'heat pool' for the hot water to draw through, so cutting down the time for warmer water to arrive at any tap.
- For split systems, do make sure that a one way valve is installed in the circuit, otherwise on cold nights the circuit can run in reverse on its own and loose heat through the roof element.