The Domestic RHI and Solar Thermal Stores

The Law of Unintended Consequences Strikes Again

The domestic RHI was structured with the intent that the complementary combination of solar thermal with other heating technologies would be actively encouraged by receiving double subsidy for the domestic hot water energy.  Unfortunately, the wording of the legislation has prevented installers using the simplest way to implement a combined system (the thermal store) because it rules out solar systems that can make even a theoretical contribution to space heating.

Thermal Stores in Hot Water

Solar thermal systems can make a contribution to space heating as well as domestic hot water (DHW) preparation, especially in spring and autumn where the days are still bright and there is a demand for space heating.  These systems are not yet as common in the UK as those for domestic hot water, but in more developed European markets such as Germany and Austria, so-called "solar combi systems" are popular.

In a thermal store the domestic hot water is heated in a heat exchanger
and the contents of the store pumped around the space heating circuit

A good way to combine solar thermal with space heating is to use a thermal store, essentially a large (typically 500 litre minimum to 1,000 litre) hot water cylinder with heat inputs from both solar and the backup heating system and with outputs to domestic hot water and space heating.  Typically the body of water in the thermal store is heating system fluid (primary water) and domestic hot water is heated on-demand in a heat exchanger as it flows to the hot tap.

Both heat pumps and biomass heaters operate well when running continuously rather than cycling on and off, so charging a thermal store is a good technical solution that improves the overall efficiency of the heat pump or biomass boiler.

Where the designer is seeking for the solar to make a reasonable contribution to the space heating, the solar panel array installed is large (around 12-18 m² for a domestic property).   The coverage of domestic hot water of such systems can be very high, 70% and above.

Where the designer is aiming for solar to mainly cover domestic hot water the panel array is smaller (typically in the range of 3  - 6 m²).  In this case there is still a theoretical possibility that the solar energy will contribute to the space heating, though in practice the system is sized with the aim of supplying 60-70% of water heating.

The current domestic RHI legislation completely excludes systems that can contribute towards space heating.  

The text in the RHI regulations defines an eligible solar system as follows:

a)     is designed and installed to provide heating solely to a single eligible property and solely for an eligible purpose using liquid as a medium for delivering that heat;

(b) meets the requirements set out in whichever of the standards for solar thermal plants specified in paragraph 1(5)(a) and (b)“eligible purpose” means, in relation to heat generated by— […](b) a solar thermal plant, the purpose of domestic hot water heating for an eligible property;

An implementation of solar where there is even a theoretical possibility of the solar contributing towards space heating is completely excluded from the scheme.

The reasoning behind ruling out solar space heating was that the domestic RHI is “deemed” – the solar energy is not measured, instead it is estimated using an approved calculation and the calculation only works for domestic hot water.

However, by ruling out any solar installation that does not solely heat domestic hot water, the domestic RHI has made the combination of complementary renewable heating technologies such as solar and heat pumps less likely. Solar thermal has lower associated carbon emissions than any form of back up heater, so every unit of solar thermal heat that can be used, whether for space heating or domestic hot water reduces carbon emissions.

Configurations where the solar is offsetting a proportion of fossil fuel space heating are also disincentivised by their complete exclusion from the domestic RHI.

When installing biomass or heat pumps with a thermal store, the additional cost to add a solar coil into the store is very low, making the marginal cost of adding solar thermal more attractive.  The domestic RHI would provide greater value for money if it encouraged, rather than discouraged such systems.

So how could the domestic RHI be changed to include solar space heating?

Two Suggestions

Two options occur, though I’d be pleased to hear of any other suggestions (please use the comments section).

First, it would clearly be possible to use a heat meter to measure the solar input into the thermal store.  Solar space heating systems cost more than solar systems aimed only at domestic hot water.  A requirement to fit a heat meter would be a relatively smaller proportion of the total installed cost and energy benefits, and houses that can fit large thermal stores are relatively thin on the ground, so it wouldn’t be too much of a cost for the scheme administrators to deal with the meter readings.

A second approach would be to allow space heating systems onto the scheme but to give RHI payments only for the domestic hot water energy provided, and calculate this with the current deeming method.  I’ve looked at this with the help of two years'  of data from a solar space heating system provided by Geoff Miller of GreenLincs Energy.  Simulations have also confirmed that the solar energy generated by a system providing solar space heating and domestic hot water is always higher than the same sized system targeted at only domestic hot water.  The RHI wouldn't be over-paying for solar heat.

The best outcome would be for it to be the choice of the homeowner whether or not to go to the expense and hassle of having a heat meter.  If they wanted the extra payments for space heating, then they would need to install a heat meter, otherwise they could claim for only the solar heat in their domestic hot water.

This has formed the basis of a proposal submitted to the Department of Energy and Climate Change (DECC) yesterday outlining how the scheme could be improved by allowing solar space heating.

The 100% Solar Powered National Grid?

Image: Viridian Solar

Is your Car the Star?

I was intrigued by a recent article in the Telegraph (of all places)  in which the writer predicted that solar will dominate global energy production.

The article extrapolated the falling costs of solar photovoltaic generation in recent years and combined this with a whistle-stop tour of technical innovations in the sector - perovskite solar cells, flow battery technology  and robotic cleaning of panels in solar farms.

The article predicted the end of the fossil-fuel era, and concluded that wind turbines would at best be a regional niche technology because their costs were static by comparison with the enormous technological strides that will continue to be made in solar.

As is common in such articles, the intermittent availability of solar power was mentioned only in passing along with the solution – energy storage technologies.

The article made me wonder about energy storage.  How much energy storage would be needed if solar were to win the race for global dominance of energy production?  Is it a realistic prospect? 

Solar power production is intermittent in three ways.  First there is the unpredictability of cloud cover and light levels during the daytime.  Second there is the more predictable effect of the sun travelling across the sky during the day and disappearing at night.  Finally there are the seasonal variations in day length and clear skies from summer through to winter.

One Helluva Battery

Let’s do some simple analysis to try to quantify the challenge.  To keep things simple, lets take a look at the following question:

‘How much energy storage would be required to hold the electricity generated during daylight hours for use overnight?”

The graph shows the half-hourly electricity demand from the UK national grid for 23rd April 2013.  I chose a day that was pretty sunny, but in spring rather than summer or winter.  The total electricity consumption that day was around 826,000 MWh.

On the same day in April 2013, the 44.5 kWp south-facing PV array on the roof of Viridian Solar’s factory and headquarters in Cambridge produced 233 kWh, or 0.233 MWh.

If we were to scale up the PV array to produce the same number of units of electricity as the UK demand that day, it would be:

44.5 x 826,000 / 0.233 = 158,000 MWp or 158 GWp

To put this figure in context, the UK government central forecast predicts installation totalling 10 GWp within a decade in its solar strategy, however the UK installed 1.1GWp in the first quarter of 2013.

If progress continues in the reduction of costs of solar modules, then it’s not so impossible to imagine that the deployment of this level of capacity over the coming decades might happen.

I've scaled up the output curve from the Viridian Solar PV array for that day as if it were 158 GWp and added it to the graph to show the timing of solar power generation compared to the timing of electricity demand.

The energy storage required for the solar output to keep the country going through the evening, night and until the solar output exceeded the demand the next morning would be 428,000 MWh or 428 GWh.

By using the current level of electricity demand we’ve ignored the possibility that energy efficiency gains might reduce total electricity demand significantly and thus drive down the amount of energy storage we would need. 

We've also ignored the new technologies emerging that intelligently move the timing of electricity use for applications that are not time critical (such as running air conditioning and refrigeration).  Such technology could change the demand curve to better fit the solar output, also reducing the amount of storage required.

However, some 40% of our energy use today is for heating and currently most of this does not use electricity but instead burns gas and oil.  This massive demand for energy will also need to be de-carbonised and it seems likely that it will require electricity to operate heat pumps.  Clearly heating demand can be reduced with better thermal insulation and solar heating panels on our buildings, but it seems that we’ll be doing well if we can hold our electricity demand flat relative to current levels.

So what does 428,000 MWh mean in the real world?

Consider Dinorwig pumped storage power station.  This massive civil engineering endeavour in Snowdonia National Park comprises 16km of tunnels, 1 million tonnes of concrete and 4,500 tonnes of steel. The turbine hall is Europe’s largest man-made cavern.  Cheap night-time electricity powers reversible turbines to pump water from a lower reservoir at the bottom of the mountain to a reservoir higher up the mountain.  At times of peak demand the water is allowed to run back down through the turbines and generates electricity.

It cost £425m, took ten years to build and was completed in 1984, that works out to around £2bn in today’s money.

It has a power output of 1,728 MW for six hours, a total of 8,640 MWh.

We’d need to build another 48 of these babies to store solar PV electricity through a 24-hour period to meet demand.  Assuming sites could be found, and that their costs were similar to Dinorwig, then the bill looks like it would be a cool £100bn.  That’s a very big number indeed, equivalent to around 2.5 times the estimated cost of the HS2 rail line.

 

EVs for PVs

But maybe there’s a cheaper way.  Maybe the answer to the intermittent nature of renewable energy is sitting in driveways and garages all over the country.

Would it be possible to use the batteries of electric vehicles to store energy from times of excess generation and release it at times of need?  What do the numbers look like?

One of the main thrusts of electric vehicle development is battery storage.  The challenge is that although most vehicles are used for relatively short journeys most of the time, occasionally we like to go away to visit family or friends further afield.  We want our vehicles to have enough range to cover these infrequent long journeys.

Here’s the capacity and estimated range of a few of the electric vehicles currently on sale.

Source: http://www.fueleconomy.gov/feg/evsbs.shtml

Most manufacturers have settled on a battery size around 25kWh as the right compromise on range and cost.  It seems likely that for mass adoption the manufacturers will need to develop vehicles with higher ranges.  However, let’s use 25kWh as a median value for the battery size in a car.

The annual average mileage for a car in the UK is 8,200 miles, or 22.5 miles/day.

Taking the average miles/kWh from the table means that to meet the average mileage a battery of only 6.3kWh is required.  On most days, the car has 19kWh of excess capacity that is only needed for those unusual, longer journeys.  If, through smart metering and information technology, this excess storage could be made available (perhaps yielding a small income to the car owner), how much storage could it provide?

There were 29.1million cars on the road in the UK in 2013.

If we assume that the entire UK car fleet changes over to electric vehicles then the excess battery storage this would represent is 515,000 MWh, higher than the amount that I calculated would be needed to spread PV generation out through a 24-hour period.

Hold on a Minute

Before we get too carried away, there are holes in this rosy picture of the future that need to be mentioned. 

We’ve only thought about storing energy from one (sunny) day to the next, but we need to solve the problem of having energy available in winter too, or on days where the sunlight is lower. 

One option is to install a massive excess of solar PV – many times more to meet demand in winter, 90% of which would then be idle on sunny days.  Solar panel prices would have to drop spectacularly to make this approach viable.

Another concept, popularised by the Desertec Foundation is to locate the solar panels in a place where it’s sunny more of the time and day length doesn't vary so much.  We then build low-loss power cables to connect from the desert to population centres.
 
It seems to me that wind, wave and tidal power generation will have a prominent role to play, due to their complementarity to solar – the wind still blows at night and average wind speeds are higher in winter.

Whatever the future holds, its clear that energy storage has a significant part to play, and how ironic if the car, which did so much to drive the development of the global petrochemical industry, will play a central role in bringing our reliance on fossil fuels to an end. 

 

Merton Rule Lives on

Housing Standards Review Steps Back from Brink

Image: Viridian Solar

The Department for Communities and Local Government (DCLG) has concluded its Housing Standards Review and contrary to expectations the so-called Merton Rule, whereby Local Authorities can specify that new homes generate a certain portion of their energy use from renewable sources is retained.

 Part of the government's 'Red Tape Challenge', the Housing Standards Review (HSR) was wide-ranging and covered issues from wheelchair access in new homes, their consumption of water and use of energy.  The starting point of the review was that there has been a proliferation of different standards and that this is costly for both housing developers comply with and for Local Authorities to police.  DCLG proposed a number of ways in which it might simplify matters.

The consultation published in the autumn contained a serious threat to the deployment of renewable energy in new homes.   (See my earlier blog and infographic). 

After a series of changes in which the energy efficiency of new homes improved rapidly, progress has completely stalled since the coalition government came into power.  As a result, current building regulations can easily be met without renewables.  The only driver to encourage developers to use renewables in new homes is that many Local Authorities require it as a planning condition (often called the ‘Merton Rule’ after the first Authority to pilot the idea).  They can do this because they were granted the power in the Planning and Energy Act 2008.

Many Local Authorities have adopted planning policies like this as part of their Local Plans with the goal of creating local skills and supply chains, mainstreaming renewables and encouraging their wider adoption.

The HSR consultation document proposed to remove this power, potentially leaving renewable energy in new homes out in the cold until the building regulations reach 'Zero Carbon' and this feeds through into actual projects – potentially as late as 2022.

The Impact Assessment that accompanied the HSR consultation gave a clear indication of what was influencing DCLG’s thinking.  The anticipated ‘savings to industry’ only counted the reduction in the costs of housing developers.  The business lost by the renewable energy industry was not considered, and nor were the savings on energy bills for the householders. 

 Housing developers were offering a narrative that was both simple and attractive to the politicians:

 "Free us from these unnecessary costs and we'll build a way out of this recession."

It was very clear who was in the driving seat.

As the civil servants at DCLG worked their way through the responses to the consultation and pondered their conclusions the country was gripped first by a debate on the affordability of energy bills and then by endless rain, flooding in Somerset and politicians in wellington boots trying to outdo one other on how serious they consider the threat of climate change.

Was it these events that influenced the outcome – the idea that DCLG could find itself ordering underwater Local Authorities in Somerset to tear up their climate change policies? 

Or perhaps the rash of record profit growth announcements by house-building companies undermined the argument that costs had to be cut to get Britain building again?

Or maybe it was the work done by the Solar Trade Association and Renewable Energy Association to present the arguments for including renewable energy as houses are built?  (Step forward Mike Landy and Leonie Greene).

 Who knows what the decisive factors were, but on Thursday 13th March, the decision was announced and the Merton Rule lives to fight another day.

Untangling the Announcement

The way the decision was announced has caused some confusion, especially because the Written Ministerial Statement said that there would be no optional additional local standards:

This caused a number of sources to wrongly report that the Merton Rule had gone.  A bit of digging shows that the opposite is true.  The new rules are to be enacted through the Deregulation Bill.  Here's the relevant section:

To understand the impact of the changes, you need to read it with the Planning and Energy Act 2008, shown below:

So it can be seen that the change only affects Section 1(1)(c), and prevents Local Authorities in England (only) from requiring energy efficiency standards higher than building regulations for houses (only).  Crucially for renewable energy sections (a) and (b) are left intact and Local Authorities can continue with planning conditions that require a proportion of energy from renewables providing a critical bridge to Zero Carbon Homes.

Attention now turns to the definition of Zero Carbon Homes, encouraging DCLG to deliver it on time and how much of the standard can be 'bought-in' rather than 'built in' through a process called 'Allowable Solutions'.



Infographic - Slow Progress on Low Energy Housing in UK

Replace or Refurbish?

What to do with older solar heating systems 

It could be so much better



I've been getting correspondence from solar installation businesses asking what the domestic RHI might mean for older solar systems, specifically ones that were never entered onto the Microgeneration Certification Scheme (MCS) when installed.  Is there any way for these to claim the Domestic Renewable Heat Incentive (RHI)?

Can you just inspect that the solar heating system is compliant with the current MCS scheme, re-commission it and register it as if you've just installed it?

Do you have to rip it out and put in a whole new one?  Would even this be allowed on the scheme?

Setting aside the fact that the intent of the dRHI was to stimulate new installations of renewable heating, and that finding a way to register an existing (and potentially working) system is not really in the spirit of things, let's have a look at the regulations and see what they have to say about it. 

MCS

A review of the MCS standards (MIS3001 and MCS 004) finds that they are silent on whether the equipment used when installing a solar system must be brand new to be registered with the scheme. The implication is therefore that an installer could go through the standard line by line to ensure that the existing installation is compliant, making changes to components as required and registering the system on the MCS database.  In effect the installer is building a system from ‘second hand’ parts, some of which happen to already be on site and fixed in place.

However, just getting MCS registered does not mean you can get the domestic RHI.  It's also necessary to comply with the eligibility requirements of the RHI scheme itself.

Domestic RHI

The domestic RHI legislation has now been laid in parliament, so it’s possible to see the basis that OFGEM will be using to create the scheme rules.

The relevant section of the domestic RHI regulations is on page 12 in section 9:

Plants used to generate heat before the first commissioning date9.—(1) The requirements referred to in regulation 3(b) are that no part of the plant which generates heat, other than any of the components listed in paragraph (2), was used before the plant’s first commissioning date.(2) The components referred to in paragraph (1) are—(a) immersion heaters and other components which solely generate heat for the purpose of heating domestic hot water;(b) supplementary electric heaters; and(c) circulation pumps.

From the above it seems that so long as the heat generating part of the installation is new, then other parts of the heating system can be re-used.  This makes sense – it would be crazy to insist that a new biomass boiler installation also had to replace all of the connecting pipes, radiators and hot water cylinder in the home.

In relation to a solar thermal system, the parts of the plant that can generate heat are:

1. Solar Collector
2. Pump
3. Immersion heater in cylinder

Items 2 and 3 are specifically excluded in the regulations.  It seems to me that to modify an existing solar thermal installation so that it is eligible to join the domestic RHI scheme, it is necessary to change the solar panels, but that all other components could be re-used.

Have I missed something?

In-roof Solar PV - Hot or Not?

Results from a study into heating effects on in-roof solar modules

In-roof solar PV may look great, but what's the trade off on energy performance?
Image: Viridian Solar and Elliott Brothers

Every solar professional knows it.  The power output from a crystalline silicon PV module reduces as it gets hotter.  PV systems installed in-roof suffer from lower ventilation rates on the shade side than modules installed on a rack above the roof covering and therefore produce less energy.

But how much less?  Is it a lot or a little?

How much of the heat is actually lost from the shade-side of a panel, compared to that from the sun-side?

I was recently in a meeting at the Solar Trade Association with a group of the top technical brains from the UK PV industry.  I asked the group to estimate the increase in annual energy yield by switching a crystalline silicon PV module from a sealed in-roof installation to an installation above roof.

The answers ranged from 1% to 14%. 

Conversations with other solar professionals have produced estimates as high as 25%.

Everyone knows it has an effect. But no one seems to know by how much.

Having a quantitative, evidence-based answer to this question is becoming more and more relevant in our industry.  As the solar market matures more and more customers for solar PV want the benefits of reduced energy bills but without compromise to the looks (and potentially re-sale value) of their properties.

In-roof systems offer an alternative that ticks the box on aesthetics for many people at a price they are willing to pay, but just how big is the trade-off on energy yield? 

Now researchers at Viridian Solar, collaborating with the Engineering Department at Cambridge University and Enphase Energy have produced an answer to this question.

The authors are aiming to publish the research in a peer-reviewed journal later this year, but a briefing document has been released summarising the experimental results.

 

Replacing Opinion with Evidence

 

The experiment is described in more detail here, but in simple terms it consisted of three steps:

 

• Build a test rig with PV modules installed in a range of situations representative of real life construction

• Understand the relationship between weather conditions and module operating temperature for each type of installation

• Use the experimentally derived temperature profiles to calculate the annual energy yield for each installation situation.

 

Test Rig

 

Clearline PV15 modules were installed in five different ways

1. Free standing on an open framework (rear fully open)

2. Above a pitched tiled roof on a metal framework (open gap between panel and tiles)

3. Integrated in a pitched tiled roof with cold-roof construction behind (batten-space ventilation)

4. Integrated in a pitched tiled roof with warm-roof construction behind (insulation between roof joists)

5. Integrated with a pitched shingle roof with plywood sarking board (module rear un-ventilated)

The images below show the roof build up for two of the pitched roof installations.

 

 

Temperature Rise

 

The graph below shows the temperature response of each of the installation types - the lines show the operating temperature above ambient as the light levels increase.  As expected, the temperature of a module with less ventilation to the shade side rise faster as light levels rise.

 

 

 

For example, at 1,000 W/m2 (a bright sunny day with sun directly onto the module) the rack-mounted module above the pitched roof was 10 degrees C warmer than the free standing module.  The integrated module in the cold roof is a further 9 degrees C warmer than the rack-mounted module.

 

Clearline PV modules have a power-temperature coefficient of -0.509 %/degree C, quite typical for a crystalline silicon module, so a reduction in temperature of 9 degrees would produce a power increase of 4.5%.

 

However it's not always sunny, and the sun isn't always directly onto the module.  In fact, in the UK irradiation levels higher than 1,000 W/m² are very much the exception and not the rule.  At lower levels of light, the temperature difference between different installation types is smaller.

 

Annual Energy

 

So, what's the answer?  What was the annual energy benefit for rack-mounted systems compared to the in-roof systems in the experiment?

 

The temperature characteristics were used with a climate file for Cambridge, UK and the power-temperature coefficient for the modules to calculate the annual energy yield for each installation.

 

It turns out that a rack-mounted module would yield 3% more energy than a roof-integrated module. 

 

 

 



 

 

 

 

 

Clearly, for some situations an extra 0.3% return on investment due to energy yield will matter, but for many domestic customers minimising the visual impact on their building will be more important. 

 

As an industry at least we now have facts to present to potential customers so that they can make an educated choice.

 

 

 

 

 

 

Domestic RHI Regulations Laid Before Parliament

58 Pages of Pure Reading Pleasure

The Regulations for the domestic RHI were laid in parliament yesterday, a significant step towards the imminent launch of the scheme. 

You can read them here:

As a result, some remaining points of clarification for solar thermal on the scheme have now been made public:
 

• Solar for space heating / indoor swimming pools are not eligible, in fact solar DHW is not eligible if it is combined with space heating or swimming pool heating.

• Air heating  (transpired) solar panels are not eligible.

• Hybrid PV-T panels are not eligible.

• The regulations refer to version 1.0 of the MCS deeming calculation (MCS024) and not the most recent version 1.1, this means that the MCS standard is immediately out of synch with the RHI and it omits the Incidence Angle Modifier from the deeming method, putting evacuated tube collectors at a disadvantage.

I suspect that the solar industry might be most aggrieved by the fact that a solar system that heats an indoor swimming pool is ineligible.  Setting aside the politics of envy, swimming pools are an ideal application for solar heating, the low temperature of the pool meaning that panels operate at higher efficiency than when preparing domestic hot water alone. 

Under the rules published yesterday, if a solar system is installed to prioritise domestic hot water with a divert to a pool once the hot water is satisfied, then the renewable heat for the domestic hot water is also ineligible.  This creates a perverse incentive to install the solar system as two separate systems, one for the pool and one for the hot water.

A simple way to deal with a complex system such as this would be to allow the owner to opt to meter heat rather than deeming it.

The scheme is not set in stone and there will be opportunities to amend it over time.  However, if the Department of Energy and Climate Change (DECC) is going to expend resources to do so, then the industry is going to have to build a strong case to support the argument that any prospective change is worth the cost and trouble. 

Should any of these omissions from the scheme concern us?  What do you think?