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.

Friday 16 November 2012

Thoughts on the RHI Domestic Consultation

The Next Big Thing?
Image: Viridian Solar
Yesterday the Solar Trade Association (STA) released its draft consultation response to the Renewable Heat Incentive domestic consultation to its membership for comment.

For anyone thinking of responding to the STA or directly to DECC, here are some things to mull over:
1.       Solar – grant or tariff

The consultation definitely leaves the door open on changing the support for solar thermal from a 7-year tariff to an upfront grant.  While the choice may seem blindingly obvious – customers would prefer a grant every time – it’s worth pausing to consider what might happen if solar thermal is treated separately to other technologies.
If solar is in a separate grant scheme, even if “branded” RHI, then it would be much, much easier for politicians to tinker with.  The recent history of solar grants (Clearskies and Low Carbon Building Programme) indicates that the temptation to continuously “improve” the system may be too great to ignore.  Even if you believe that the team setting up the RHI has the best of intentions (and I do), the nature of government is that there’ll be different people running it before too long.

For this reason, the STA is currently consulting on a hybrid – a combination of an upfront grant against cost of the cylinder upgrade and a solar thermal tariff for 7 years just like the rest of the RHI technologies.  There is a logic behind this, as the cylinder upgrade as part of a solar thermal installation results in energy savings that are not given credit in the RHI impact assessment, see my earlier blog article on the subject.

2.       Scope for Cost Reductions

The STA has estimated potential cost reductions of 35% if the solar thermal industry can achieve economies of scale (imagining a market of 150k installs/year, an increase of 5-fold from today's estimated 30k). 

The nature of solar thermal equipment is that much of the cost is fixed by the price of global commodities.  Unlike solar PV, where the main component cost is for a material that has no other bulk uses (silicon wafer),  these commodities are materials like aluminium, copper, and glass which have many other uses.  Changes in the solar market are unlikely to influence the price of these materials.

For this reason, the STA is proposing that the majority of cost reductions could come from efficiencies gained in installation rather than price reductions in materials.  It is easy to focus on the actual process of installation and to wonder whether this can be sufficiently squeezed, but that is to ignore other business efficiencies that would emerge as solar thermal graduates from cottage industry status.
Imagine a solar installation business that increases in turnover by 500% over a number of years.  Would it need 5x more people in the office, or would specialisation allow people to do business processes more efficiently?  Could it have staff which did nothing but surveying?  Would these people have to travel less distance between appointments?  How about marketing?  How much more effective would be each pound spent advertising the business when the potential market is so much larger?
Fundamentally, you have to ask the question, why should UK plc invest in solar thermal?  What’s the end goal?  Unless the RHI helps solar thermal unlock installations at lower cost so that it doesn’t need subsidy in future as energy prices increase then why bother?

Friday 9 November 2012

Heat Losses from Hot Water Cylinders

the solarblogger roots around in the nation’s airing cupboards
Tucked away in a cupboard, perhaps behind a pile of towels and bed linen, out of sight and out of mind.  The domestic hot water cylinder often escapes our attention, but it has a big role to play in the energy efficiency of our homes. 
Modern cylinders have a thick jacket of polyurethane foam insulation around the outside to reduce heat losses, but the connecting pipes can be just as important.  These provide a path for heat to escape from the cylinder.  To appreciate this fact you just have to touch them to feel how hot they can be, and how far from the cylinder connection they can conduct the heat.

Even in modern houses built to higher energy efficiency levels it is common to find a hot water cylinder installed without any insulation on the connecting pipe-work. The image (left) was taken at a visit to an eco-exemplar show home this year.  The home has a host of energy efficiency measures such as solar water heating, solar PV, and mechanical ventilation with heat recovery.  The only pipes in the cylinder cupboard with insulation on them are those to the solar panel. 
Presumably the plumbers had so impressed themselves with the job they’d done on covering the copper pipes with silver paint that they didn’t want to hide the beautiful craftsmanship. 
You can also see on the cylinder that the secondary return fitting is unused (just below the grey thermostat on the right hand side of the picture).  Instead of being closed off with a plug a short length of pipe is sticking out with a soldered end fitting.  This creates a completely unnecessary conducting rod to remove heat from the cylinder and transfer it to the surrounding air.
Did the energy assessors tick the box that said that all visible pipe-work was insulated?  I’d bet money that they did.  This is a failure of compliance as well as a brilliant illustration of a lack of awareness from installers. 

If a new build eco-showcase can look like this, what hope for millions of existing homes around the country, and how much impact this can have on their energy performance?

Show me the money

Research from the Energy Saving Trust demonstrates just how significant the heat losses from poorly performing hot water cylinders can be.

Heat Losses from Domestic Hot Water Cylinders

Source: Energy Saving Trust, In-situ monitoring of efficiencies of condensing boilers, June 2009

During the heating season, these cylinder heat losses are warming the house and reducing the space heating load.  For new homes with ever higher levels of insulation the heating season is becoming shorter and shorter, but for most homes you can safely assume it lasts half of the year.  So the other half of the annual loss is wasted energy, and this is shown in the last line of the table.

To put these numbers in context, the energy to heat the water demand for a four-person household is around 2,140kWh/year.  The average cylinder losses in the study add half again to the hot water load, the worst performing cylinders in the study double it.
Of course, it’s impossible to reduce the losses to zero, but what might be achievable?

Well, hot water cylinder manufacturers test and declare the heat losses from their cylinders. When you take this figure and adjust it for the fact that the cylinder doesn’t spend all day at 60C you find that a typical 210 litre modern cylinder loses around 355kWh/year, so that’s only 178kWh/year of wasted energy taking the heating season into account.  Conclusion: a modern, well-insulated cylinder saves 750kWh/year compared to  the average found in the study.
Most solar installations involve the replacement of a hot water cylinder and with upcoming changes to the MCS installation standard for solar thermal, the solar thermal installer will have to ensure that all pipes and fittings are well-insulated (not just the solar ones). 

The energy savings that come from improving the hot water cylinder as part of a solar heating installation are not acknowleged in the MCS energy estimate and do not count as renewable heat for the forthcoming Renewable Heat Incentive, but they are real and they are significant.

Sunday 4 November 2012

Who's Installing Solar PV?

In recent months I've heard a number of people saying of the solar PV market in the UK that "the domestic market is dead" and "commercial installations are where it's at".   Do the statistics support this?  Let's have a look.

The two pie charts in the graphic below shows the proportion of PV  installed under the Feed in Tariff (FIT) by the size of the installation.  It covers two six month periods, October 2011 to March 2012 and April 2012 to September 2012.  The second period is post-April, after which the government had largely set in place its measures to slow down what was rapidly becoming an unaffordable incentive scheme.  The earlier period spans three peaks caused by the race to complete installations before successive "improvements" to the scheme came into force. 
Chart showing how the solar PV market has developed in the last 12 months in the UK
As the tariff levels have reduced, the savings from own-use of the electricity generated become a more and more important proportion of the financial return. That's because if you use a unit of electricity generated you save around 12-15 pence, if you export it you get 4.5 pence.

Commercial buildings tend to use most of their electricity during the daytime - powering computers, industrial processes, lighting and air-conditioning - at least five days of the week, and sometimes seven. This matches periods of maximum PV generation very well.

By contrast, domestic properties tend to use electricity in the evening, so less of the electricity generated is is used in the building and the financial rewards are likely to be smaller. Installations in the size range of 10 to 100 kWp tend to be commercial scale installations on factory roofs, farm buildings, offices and schools, and this segment of the market has increased its share of the total. It hasn't grown, but it has shrunk much less than other sectors.

However, this increase in share hasn't been at the expense of  domestic scale installations, which have held steady at around 70% of the market and still represent the lion's share of installations by kWp as well as by number of installtions. Instead, installations in the 100-5000 kWp range now take less share of new registrations under the FIT.

So, in answer to the question, do the statistics support these views, the answer is both yes, and no.  Commercial-scale installations have definitely become a more significant part of the total, and are likely to continue to do so as electricity prices rise and tariff levels decline, but the domestic installation market is far from unimportant.

Friday 19 October 2012

Hybrid PV-Thermal Solar Panels - A Good Idea?

The following article is a summary of a more technical paper, which can be downloaded here:

Solar Thermal Panel Operating Temperature During a Calendar Year

Working for a manufacturer of both solar thermal and solar photovoltaic (PV) panels, I am often asked why we didn't combine both technologies into a single panel.  Then we wouldn't have had to spend so much time and effort making our PV and themal panels match so they look great together on the roof.  And what's more, (they continue), a hybrid PV-Thermal (PV-T) panel is better because the solar thermal removes heat and cools the PV cells to a lower temperature where they work more efficiently.

I'm going to share with you some of the work we did that informed our decision to keep solar thermal panels and solar pv panels separate. 
You see, the notion that the solar thermal part of a PV-T panel keeps the PV part of the panel cooler ignores the whole point of a solar heating system - for it to be useful it must increase the temperature of something.
The most common "something" to be increased in temperature is a tank of water for use washing and bathing in a residential building.  For sure this starts the day at a cool temperature, but if the 'T' in your PV-T panel is to be of any use, then it will finish the day at a warmer temperature.
The image at the top of the page shows the operating temperature of a solar thermal panel heating domestic hot water in three minute time slices for a whole year.

As each day starts (top of the image), the panel is working at low temperatures (blue/green colours), but for days with good light levels, the hot water cylinder heats up and the panel has to work at higher and higher temperatures (red/orange colours) to keep on adding heat to the hot water cylinder.  The higher temperatures reached as we move from January (left of the image) to the longer days of the summer (middle of the image) are also evident.

Also of great relevance is that the thing you are heating often has a maximum allowable temperature.  For example, when heating a hot water store it is common to stop at 60-65C to avoid scalding risks.  Once this happens, the solar thermal system is not circulating the "coolant" to the panel and the insulation that makes it thermally efficient means the panel gets very, very hot - temperatures around 220C are common.  Aside from the effect of these super-high temperatures reducing the efficiency of the PV cells in a hybrid panel, the effect of such temperature cycling of the PV cells and solder connections seems unlikely to be beneficial to the panel life expectancy .
By contrast, here's the temperature for a south facing PV panel for the same year of weather data using the same temperature colour map:

Temperature of a PV panel over the same calendar year of weather data

Like the thermal panel the PV panel starts cooler in the morning (top of the image) - at ambient temperature, then raises to the afternoon as more and more light falls upon it, before falling again as we progress to the evening (bottom of the image).

It's clear just comparing the two images visually that a solar thermal panel runs hotter than a PV panel, but here is a plot comparing the temperature of the two panels over the year:

Plot showing when a thermal panel would be cooling (blue/purple)
or heating (orange/red) a PV-T hybrid panel

Blue and purple pixels show times when the solar thermal panel is operating at a temperature below the PV panel, and would therefore be cooling a PV-T hybrid panel.  Red and orange pixels indicate that the thermal panel is operating at a temperature above the PV panel and would therefore be actually heating a hybrid panel and diminishing its electrical output compared to a stand-alone PV panel.
White pixels are where there is little difference.  Black areas are where the solar thermal system is not circulating, either because the panel is cooler than the hot water cylinder or because the hot water cylinder is at its maximum temperature (65C in this case).

Note that the red pixels are where the 'T' is heating the 'PV' to a temperature more than 10C higher than it would be in a standard PV panel.  This corresponds to a reduction of electrical power output of more than 5%.

It is possible to imagine a solar heating application which would keep the temperature lower for more of the day.  For example PV-T panels heating water in a swimming pool would operate at 30-40 degrees all the time. 
For the majority of applications though, the intuitively appealing idea that the sum of the whole is greater than its constituent parts turns out to be a mirage.  A heat-haze if you like.
This article is a summary of a more technical paper, which can be downloaded here:

Saturday 22 September 2012

So What's it Worth? - Domestic RHI for Solar

It's speculative, but at least it's a stick in the ground.

This week, the UK government published its consultation on the domestic stream of the Renewable Heat Incentive (RHI), (read the solarblogger's summary here).

The proposed tariff figure for solar heating is 17.3pence per kWh for seven years - but what does this mean in terms of the potential value of the RHI to a household?  the solarblogger does the maths.

The energy saved by a solar water heating system is currently estimated for customers by their installer using a calculation published by the Microgeneration Certification Scheme.  There is also a consultation in progress for this standard, including updates to the solar energy calculation.  I have used the proposed new calculation method.

Area of Solar Collector
Energy Saving
Solar Energy Qs
The main driver of hot water use in a home is the number of people resident there.  I have selected a typical solar panel area and hot water cylinder for the occupancy, and calculated the Solar Energy (input to the hot water cylinder), and the Energy Saving (gas saved at the boiler).  The Energy Saving is the higher of the two due to the (in)efficiency of the boiler.

Assuming that the RHI is paid on the renewable energy input to the hot water cylinder, the value of the RHI and fuel savings are shown below.

Potential RHI Solar Payments for Off-gas Property

Annual RHI Payment
Annual Own Use Saving
Total RHI Payments
Fuel Saving
(7 years)
(7 years)
(7 years)
 £  102.13
 £         61.49
 £     714.88
 £       430.44
 £    1145
 £  130.02
 £         78.29
 £     910.12
 £     548.00 
 £    1458
 £  190.16
 £       114.50
 £  1,331.14
 £       801.51
 £    2133
 £  221.18
 £       133.18
 £  1,548.28
 £    932.25
 £    2481
 £  277.96
 £       167.37
 £  1,945.72
 £  1171.56
 £    3117
 £  307.21
 £       184.97
 £  2,150.44
 £    1,294.82
 £    3445

For a property heating water with an oil-fired boiler, the value of the RHI plus fuel savings over the seven year period of the RHI would be around £2,500 for a 4-person household (expressed in 2012 prices).  Of course, the solar heating system would continue to deliver fuel energy savings after year 7.

 Potential RHI Solar Payments for Gas Heated Property


Annual RHI Payment
Annual Own Use Saving
Total RHI Payments
Fuel Saving
(7 years)
(7 years)
(7 years)
 £  102.13
 £         38.43
 £     714.88
 £     269.03
 £  984
 £  130.02
 £         48.93
 £     910.12
 £     342.50 
 £  1253
 £  190.16
 £       71.56
 £  1,331.14
 £     500.94
 £  1832
 £  221.18
 £       83.24
 £  1,548.28
 £     582.66
 £  2131
 £  277.96
 £       104.60
 £  1,945.72
   £      732.22 
 £   2678
 £  307.21
 £       115.61
 £  2,150.44
 £    809.26
 £  2,960

 For a property heated by gas, the incentive is slightly lower because the saving from fuel use avoided is lower- gas is a cheaper fuel than oil, but (again expressed in 2012 prices) the value of a solar heating system to a 4-person household would be around £2,150 over the first seven years, with energy savings for another 20 years after.

But is it enough?

Given that a solar heating system will cost between £4,000 and £5,000 installed, no-one is going to get rich on this scheme.  On the other hand, this level of financial assistance definitely represents a step-change in the level of government support for a very popular, well-understood and readily installed technology.

But is it enough?  What do you think?  Post a comment below.


Gas 5p/kWh, Oil 8p/kWh
All prices are 2012 values, no fuel price inflation added.

Solar panel - high performance flat plate
Orientation - South at 30 degrees pitch, no shading

Assumed that RHI is paid on Qs - solar energy input to hot water cylinder
Own use savings based on a modern condensing boiler, but using a "summer-biased" efficiency of 77%, according to draft MIS3001 v3.0.