Understanding Solar Hot Water
We all use hot water virtually everyday for washing, cleaning and in the winter even heating. Whether you are looking to invest in solar hot water or just wishing to learn more about solar heating this page provides you with a straight forward explanation of hot water systems.
Water for domestic use is normally heated by electricity or a fuel burning boiler (gas/oil/diesel).
Electricity heats water by means of a resistance immersion element which is positioned inside the tank, directly contacting the water.
Electric resistance heating is very efficient, but is slow, taking normally around 3-4 hours to heat up a full tank of water to the target temperature. It also uses electricity, commonly produced from coal fired power plants, and so it is being phased out and even banned in some regions, replaced with cleaner gas and of course SOLAR!
How Much Energy is Needed?
An average household of 4-5 people will use around 300L of hot water per day. Given that the temperature of the hot water in the storage tank will normally be about 60oC / 140oF, the actual amount of hot water used at the tap could be 30% more, depending on how much cold water is mixed in and of course the cold water temperature.
The amount of hot water for such a household could easily be reduced by 25-40% by installing a low flow shower head. A tap flow rate of 8L/min is more than enough for a good strong enjoyable shower – if you have the right shower head!
The amount of energy required to heat the water in the tank to 60oC / 140oF will depend on how cold the water is to start with and this will change through the year. Normally it will be within the range of 5oC – 20oC / 40oF – 68oF. So in the winter when the water is cold, more energy is required to achieve the same final temperature.
Solar Heating Basics
The amount the tank temperature increases with solar input depends on how much hot water remains in the top of the tank, and therefore how much “cold” volume is available for solar heating. This is why in tanks A and C, the temperature rise is moderate, as the full tank volume is being heated.
In tanks B and D, only the bottom half is being heated and therefore the temperature rise is much greater. In tank D, the bottom is first heated to 60oC / 140oF, then the entire contents is further heated up to 75oC / 167oF, at which point the controller turns off the pump to avoid further heating of the tank (depends on controller configuration).
The aim for any solar heating system should be to use all of the energy produced by the collector. Heating a tank full of cold water, as shown in A & C will result in more total heat production than for B & D, as the collector will be running cooler. Furthermore with a small volume of cold water to heat, the tank may even hit maximum temp (as shown in D), turning the pump off and potentially wasting good afternoon sunlight. For this reason BIG is generally BETTER when looking at storage tank sizing.
Having a dedicated solar tank is advantageous, and for this reason a gas boost system is always preferable over an electric boosted tank system.