Sunday, 16 December 2012

Turn up the Volume - What is Dedicated Solar Volume and why does it Matter?

Engineers and designers working with solar heating systems have wonderful simulation software available to calculate the energy that will be produced by their designs.  However, the seductively accurate predictions they can produce utterly fail to take into account the biggest single influence on most solar heating systems – the people using them.

You see, people don’t always behave like engineers expect them to.

A study by Viridian Solar monitored the performance of solar water heating systems in six homes.  The houses were rented, and the two landlords had paid for the installation.  Householders received instructions on how to get the most out of the system, and being in the social rented sector had a higher motivation than average to control their expenditure on fuel.  And yet...

More recently, a larger study of around 100 homes by the Energy Saving Trust was published.  The houses were privately owned in this case.  They had spent their own money on the solar installation, and were “Early Adopters” of the technology so it’s hard to imagine a more engaged and motivated set of users.  And yet...

How to get the Least from Your Solar Heating System

Both studies found that a significant proportion of the households were not controlling their back up heater relative to the timing of their hot water use to get the most from their solar heating.  The diagram below explains.



In any solar installation where the back-up heater (for example a boiler) heats the same volume of water as the solar panels, the timing of when the back-up heater fires will influence the solar energy collected. 

The most common solar water heating set up is shown above – a twin coil cylinder. 

The back-up heater heats the water inside the hot water store (or cylinder) by pumping central heating fluid through a coil of pipe inside the cylinder.  Water surrounding the coil is warmed and heat is transferred around the cylinder by convection – rising currents of warmer water.  Since hot water rises, the coil can only heat the volume of water above it and a volume of water below is left unheated.  This unheated volume that the back-up heater cannot heat is called the “dedicated solar volume”.

The solar panels heat the cylinder from a second coil of pipe at the bottom of the cylinder, and so can heat the whole height of the water in the cylinder.

Most domestic buildings have a fairly well-defined pattern of hot water use, with periods of highest use in the evening and/or morning.  If the back-up heater is timed to come on and then  switch off before the period of high hot water use starts, then the hot water is taken out of the top of the cylinder and replaced with cold water at the bottom.  When the sun comes out and the solar panels start to work, there is the largest possible volume of cold water available in the cylinder for them to heat up.

If instead the back-up heater continues to run during and after the period of hot water use, then the whole top part of the cylinder will be at its maximum temperature.  Fossil-fuel fired heaters will heat the water so quickly that it only needs to over-run the period of water use by 20-30 minutes and the water will be hot again. When the solar panels start to work, the only cold water is at the bottom of the cylinder – the dedicated solar volume. 

Once this water is heated to the maximum safe temperature, there is nowhere else to put the solar energy, the solar panels must switch off, even if there is plenty more energy available that day. 

This reduces the amount of energy saving from the solar panels by an amount that can dwarf the impact of other factors such as whether the panels face due south.

The Importance of Dedicated Solar Volume

If even the most motivated of solar system owners are prone to timing the back-up heater to reduce solar yield, what hope is there once the forthcoming Renewable Heat Incentive moves solar heating into the mainstream? 

Fortunately, there’s a very simple answer.

The larger you make the dedicated solar volume (that bit of the cylinder that the back-up heater cannot heat) the less sensitive the system becomes.  A system with dedicated solar volume approaching the daily hot water use of the household would be much less affected by poor use of the back-up heater.

Products are available that allow the conversion of existing cylinders to accept a solar heat input.  These have the advantage of being more economic, but the disadvantage is that the dedicated solar volume is zero, since the boiler heating coil is at the base of the cylinder.  Poorly timed use of the back-up heater in such systems will reduce the available capacity in the cylinder for solar energy down to nothing effectively wiping out the solar energy savings on that day.  Such products have a place, but only where the user is completely bought into the fact that they must closely control their heating system.

Some manufacturers have proposed a boiler interlock to improve matters.  If the solar panels are heating the cylinder, the interlock prevents the boiler from firing.  The reason this doesn’t solve the problem is that if the hot water is used in the evening or early morning, there is ample time for the boiler to re-heat the cylinder before the solar panels start to work.

The only sure-fire way to ensure that a solar water heating system really delivers on promised energy savings is to ensure adequate dedicated solar volume. 


  1. Stuart, I'd have to disagree about software not allowing for dedicated solar volume... TSol allows you to program in tapping heights, sensor heights, and temperature levels. I'm not an expert on Polysun, but I'd be surprised if the "Designer" level software didn't allow this as well

    I do agree with the rest of the post though : )

    I'm wondering how long before instantaneous water heaters start becoming more common to allow cylinders to be completely dedicated to solar storage, in conjunction with the "fresh water" stations so popular in Germany (also usefully removing legionella risk if required).

    Unfortunately another layer of expense that the UK market so often seems resistant to.

  2. Stuart, a very well presented and a valid argument. I have experience of tenants changing the back up heating timings as well. The problem seems to be that they only need to have one cold shower and the timings are overidden permanently.

    I would like to add a key observation: the real issue is control of back up heating not dedicated solar volume. It is only because the technologies for control of back up heating are inadequate that we require such a crude and expensive method for ensuring solar energy is directed to cold water ..... the dedicated solar volume.

    As you say, where users are 'bought into the fact that they must closely control their heating system' dedicated solar volume is not required.