Monday 31 December 2012

Just What's Wrong with the Commercial RHI?

Just over one year in, how’s the commercial Renewable Heat Incentive doing?

In November 2011, the Department of Energy and Climate Change (DECC) scored a world’s first, launching a kind of Feed in Tariff for heat.  Called the Renewable Heat Incentive (RHI), the scheme pays out for each kilowatt hour (kWh) of useful heat generated by a range of low carbon heating technologies including solar thermal, heat pumps and biomass boilers.

In its first phase, the scheme is open only to non-domestic installations, with a scheme for householders to follow in summer 2013.

(For more on the domestic RHI, read my earlier article: “The Domestic Renewable Heat Incentive for Solar”)

How’s it doing?

So, are civil servants from other countries going to be beating a path to the UK to learn how its done?  On the basis of the performance to date, we don't need to worry about putting more border agency staff on at Heathrow.

OFGEM are administering the scheme, and you can get hold of performance data here.

And the winner is...

The chart shows the number of installations of the three most popular technologies, although it can be seen that the scheme is having mixed success.

The scheme has supported 679 installations of biomass boilers, but only 36 ground source heat pumps and 33 solar thermal systems since it began over 12 months ago.

In terms of eligible heat generated, the skew towards biomass is even more marked with 65.5 GWh of biomass heat generated, compared to a total of 0.9GWh from all other technologies.

To put this in context, the Feed in Tariff stimulated 30,000 installations in its first year.  According to DECC's heat strategy, electricity for lighting and appliances is only 8% of final energy use whereas heating is 46%.  

So the problem isn’t so much runaway take up of biomass as the unpopularity of the scheme as a whole.

How to Fix It

Beyond the obvious first response of making the tariffs higher, is there anything else that OFGEM and DECC could do to improve the success of the scheme?

Here are some suggestions.

  • Increase Awareness  - develop case studies with businesses that have benefitted from the scheme and work with the trade associations to disseminate the case studies to other potential customers.
  •  Streamline the application process – one of the most common problems with applications has apparently been the quality of the schematic drawings showing the location of heat meters.  OFGEM could develop standard schematic layouts, which applicants could select from, rather than commissioning their own drawings. 
Do you have any comments on the scheme or how it could be improved for solar thermal and heat pumps?  Perhaps you have direct experience of making an application?  Please make a comment below.

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. 

Tuesday 4 December 2012

How does demand for electricity vary?

We’ve all heard about the surge in demand for electricity that comes at half time during the big match as everyone brews up a cup of tea, but how much higher is the peak demand for electricity than the lowest level on a normal day?  Have a guess.
A lot is written about the “how much” of energy use, but the “when” is also of great interest –particularly in relation to energy supplies like photovoltaic solar power and wind power that you can’t just switch on and off.
National Grid (the company in charge of getting electricity from the generator to the user in the UK) publishes detailed statistics on electricity demand.

 Electricity Demand Follows a Predictable Daily Pattern


The first graphic shows a plot of the half-hourly instantaneous power consumption of the UK electricity grid for 5th January and 29th June 2011.
Both are working days, but obviously one is in the depths of winter, and one in summer.  What’s striking is how similar the curves are. 
From about 6am, the country begins to wake up and switches on heating, kettles and toasters.  Power demand rises to about 10am as people head off for work and school, and then flattens for the rest of the working day.  The daily peak in demand follows at about 5:30pm as people arrive home and switch on domestic appliances, while offices and shops remain open.  This peak is much more pronounced in the winter as people are more likely to be indoors and have the lights and heating on.
From 5:30pm, demand falls as the offices and shops close and then people turn off lights and appliances and head off to bed.
But demand doesn’t fall so much as you might think through the night, many industrial processes continue 24 hours, and people time the use of appliances and storage heaters to take advantage of cheaper overnight electricity.
 In fact the ratio of highest to lowest demand on these two days is only 1.6.

...and a Weekly Pattern


The second chart shows the energy demand from two weeks from the summer of 2011.  A saw-tooth pattern is evident, with lower demand at weekends as offices and businesses are closed.  Weekend demand is around five to ten percent lower than week-day demand.

...and a Seasonal Pattern


The third chart shows the daily energy demand for the whole of 2011, and shows the difference between summer demand and winter demand.  Electricity demand in the winter is around 30% higher than the summer.  Summertime air-conditioning loads are more than offset by the increased use of electrical space heating, lighting, and clothes drying in winter.
So, was your guess close?  If it wasn’t you’re in good company.  A straw poll around the office yielded guesses that ranged from peak demand being six times more to a hundred times more than the lowest demand on the same day.