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. 

 

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.

 

 

 

 

 

 

Consumption Assumption – The Proportion of Solar Electricity Used in a Home

The answer is 50%, now what was the question?

Three very different distributions,
each with the same average - 50%

If you have a solar PV system fitted to your home, then the amount of electricity you generate is changing all the time as the sun travels across the sky and clouds come and go.  Your electricity use also changes as you switch electrical appliances on and off.  

If solar electricity production exceeds your electricity use at any moment, power flows out of your home to supply the electricity grid (called export).  If, on the other hand, your electricity use exceeds the electricity produced by the solar panels then all of the solar electricity is used in your home and supplemented by power drawn from the grid.

Over the course of the day your electricity meter will record the flows of electricity into your home (for which you will be billed). If you can meet your electricity needs with a unit of electricity from your solar panels, then you save yourself buying that unit from your supplier.  

This saving on electricity bought represents an increasingly large part of the financial benefits of owning a solar PV installation.

So what percentage of the electricity that is generated by a solar PV system is actually used in a building?

It seems that the UK solar industry has found a rather simple way to answer this question:

Q: Retired couple at home during the day, 1kWp system

A: 50%

Q: Working couple no-one at home during the day, 1kWp system

A: 50%

Q: Working couple no-one at home during the day, 4kWp system,

A: you guessed it, 50%

For smaller residential solar systems claiming the Feed in Tariff (FIT) , the government decided that it wouldn't measure exported solar electricity, only generated electricity.  However, the design of the scheme was to reward people with money for everything they generate, plus a little more for the energy that is exported.  So it came up with a way of calculating the exported energy. A figure of 50% is multiplied by the generated energy to calculate a value for the exported electricity. 

The industry has embraced that figure as if it represents some kind of truth written on tablets of stone. Every presentation of financial benefits from the FIT I have seen assumes that the household will use 50% of the energy it generates. 

When the UK FIT was paying people £0.43 for each unit (kWh) of solar electricity generated and electricity cost £0.14/kWh all that really mattered to the financial calculation was how much you generated, not how much of it you used yourself (called self-consumption). 

PV prices have fallen dramatically and to keep the financial benefits consistent the FIT now pays new installations much less, about £0.15/kWh generated.  In the meantime electricity costs have risen to around £0.15/kWh.  Self-consumption is becoming the greater part of the financial benefit. 

To illustrate this, let’s consider a house with a 2kWp system installed. To keep the maths easy, we'll assume its in a very good location and generates 2,000 kWh per year.  What happens to the financial return as the proportion of energy used in the home changes?

How the make-up of Feed in Tariff Benefits has Changed Since Launch
Savings on electricity bills forms a much larger proportion of the financial calculation

When the FIT launched, any inaccuracy in the self-consumption figure was much less significant than today.  The value of the savings on the electricity bill represented only 14% of the total benefits (assuming this figure of 50%).

Now the proportion of the benefit that comes through electricity savings is 30%.  Inaccuracies in the self-generation assumption have a much bigger impact on the financial returns calculated.  If solar prices continue to fall then the FIT will fall too, and this trend will continue.

As an aside, the table above is absolutely not an advert for so-called PV switches that divert solar electricity to an immersion heater in your hot water cylinder .  Once you ‘degrade’ high value electricity to heat, you reduce its value to that of the cost of the fuel would have used to heat the water.  In 80% of homes this is gas, so it’s not 15p/kWh you’re saving, but more like 5p/kWh. 

(See the outspoken guest blog from Tom Seppings of solaplug for more on this point).

Far better to use it to power electrical appliances if possible, or store it in a battery, an emerging technology already causing great excitement in the industry.

Your Guess is a Good as Mine

How good a guess is 50%?  What evidence was the government’s choice based on?  If anyone has information on this, I’d love to hear it. Please post a comment at the bottom of the article.

Let's pause to consider averages for a moment. The graphs above show three 'distributions', each of which has an average of 50%. Only the top one (1) would strongly justify using the average value for all customers. 

My hunch is that the reality is most like the bottom distribution (3), with two distinct groupings - households with people at home during the day, and households where people are out at work and school during the day. 

I also suspect that the 50% figure was arrived at when the average PV system size was much smaller than it is today. Falling prices have brought larger installations within the reach of more people (capped by the 4kWp tariff band limit). In 2011, the EST had the average system size as 2.75kWp.  Today it is 3.75kWp. The larger the system, the larger the midday energy peak, and the less likely it is that there's sufficient instantaneous electricity use in the home to mop up the solar electricity. 

Yet the industry persists with presenting financial benefits based on this 50% figure.  

Does anyone know of any work that has been done on the profile of electricity demand during the day in homes with differing types of occupancy?  How could the solar industry offer its customers a better prediction of how much solar energy they will consume themselves?

Do you think solar PV systems will become smaller to increase the proportion of self-consumption as levels of FIT support falls and electricity prices rise?

Let's hear what you think - post a comment below.