Stratification / de-stratification

The layering of hot water on colder water.

In a solar hot water storage tank, the hot water is layered on top of colder water. As a result of this layering, or stratification, when the hot tap is turned on the water at the TOP of the cylinder, which has been topped up in temperature by gas heating delivered by the upper coil, is allowed to emerge from the taps. Meanwhile water enters the BOTTOM of the storage tank to replace the water which has been drawn off. In a solar hot water cylinder the solar energy is transferred to the cooler water surrounding the solar coil in the lower part of the cylinder which raises its temperature. As the stratification of the water volume stored within the cylinder occurs due to differences in temperature aided by the orientation of inlet and draw-off ports the solar heated volume is pre-feeding the boiler-heated volume/proportion of the cylinder. In a well designed system, for much of the year the solar heat input is sufficient to encourage solar heated water to exchange with water in the boiler-heated proportion of the cylinder due to temperature difference, and thus for the solar heating coil to heat the entire cylinder volume to above the boiler coil control stat temperature. In this case there will be no need for further heating by the boiler and the solar will be providing the entire hot water heating necessary for that cylinder. Furthermore the system may be sized to achieve more water heating than this in these months and so move to heat a second, pre-feed cylinder, which will fill the bottom of the already hot twin coil cylinder with pre-pre-heated water. The traditional single coil cylinder is replaced by a dual coil one, so that heat input can come from a greater number of sources which each have separate systems and separated heat transfer fluid. The increase in the number of coils also allows for greater use of stratification and efficiencies gained through sequential heating: solar cylinders are usually narrower and taller, which allows for better stratification of the water temperature – though this is not always the case. In many circumstances, the water at the top of the cylinder (this volume being referred to is normally between 1/3 to 3/5 of the cylinder volume, from the top down: the remainder is considered solar-dedicated volume.) is enough for immediate needs. However, if a full tank capacity is needed then a circulating pump can be turned on (manually or by timer) which mixes, or de-stratifies, the water in the cylinder, enabling the full volume of water to be heated by the top coil. The operation of de-stratification dramatically reduces the effectiveness and efficiency of a solar system’s heat input capacity fitted to that cylinder. The boiler is also heating a much larger volume of water than it would be subjected to if stratification was allowed to occur.

De-stratification, as well as being used to eliminate the very small potential for bacteria existence in cooler parts of a cylinder, minimises the size of a cylinder that can service the demand it is subjected to or makes up for a cylinder which is much too small to cope with peak demand periods (due to the differences in recovery rates which are about one hour maximum for boiler heat input and a day per cylinder volume for solar input). A cylinder that has been completely de-stratified or is heated completely by the boiler by incorrect use of the cylinder ports will result in a solar system which is seriously incapacitated and will not perform as designed. Also, the fuel costs for hot water provision are likely to be higher than for a standard size cylinder as a solar dedicated volume will now become part of the heating load which the gas fired boiler will have to heat.