Sections

Services

Information

UK United Kingdom

When the sun don’t shine, the power don’t flow … or does it?

Renewable energy seems to be on a roll. One million Australian homes have rooftop solar cells. There’s so much renewable energy it’s reducing wholesale electricity prices. But then, that old chestnut pops…

When it’s hot, the sun is shining. That’s good for solar power. Flickr/jimwhimpey

Renewable energy seems to be on a roll. One million Australian homes have rooftop solar cells. There’s so much renewable energy it’s reducing wholesale electricity prices. But then, that old chestnut pops up: reliability. How do we make energy when there’s no sun?

When the heat is on, the sun is shining

The electricity market is effectively two separate markets. There’s the market for energy: the coal, gas, operations and maintenance component of running the network.

Then there’s the market for capacity. To serve our electricity needs, we build enough power stations to supply our highest imagined electricity needs, then let some or most of them sit idle for all the parts of the year when we’re not using quite as much electricity.

We tend to think about the cost of power as being related to things such as coal and gas prices, or operation expenses; but a large part of the cost of electricity is simply the cost of money involved in having standby generators ready for that one moment when we all want to use electricity at the same time.

This creates a problem for the owner of a solar or wind farm. Because they can’t guarantee capacity due to the vagaries of weather, in effect, they can only sell energy

But distributed solar is different. The time when we all decide to use a lot of electricity at the same time is when we all turn our air conditioners on mid-afternoon on a scorching hot summer day. At that time, the sun is clearly shining. For distributed solar to be interrupted, it would have to be overcast over an entire capital city, in which case the temperature wouldn’t be that high and we wouldn’t actually need spare capacity.

Alongside the growth of solar installation, there has been a growing enthusiasm for sharing data on solar output. One such site is PVOutput.org. This large sample of real-time solar system behaviour under real world conditions is combined with data from the renewable energy regulator on installation rates. It allows real-time solar generation in the National Electricity Market to be calculated.

Using this data we can have a look at how renewable energy actually handles peak demand.

During the demand peak on top five highest demand days in each state, solar reduced demand by at least 21% of its installed capacity in each state. Surprisingly, Victoria and South Australia are actually better at supplying peak demand with solar. The low latitude creates long days and daylight saving time shifts consumption earlier into the day when the sun is higher in the sky. Less surprising is that north-west and west facing solar are better at meeting peak demand.

Moving from gas to solar

It’s even possible to put some rough numbers on this ability to meet peak demand. A lot of the recent work on the value of distributed solar has been conducted by regulatory bodies as they try to determine a fair price for solar feed-ins.

The regulators and their consultants all fall into the error of assessing distributed solar as only an energy source, without regard for solar’s value as capacity.

The easiest way to think about peak capacity is to value it the same as a gas turbine. In fact distributed solar has several advantages over gas turbines. Solar can be installed incrementally, as needed. New transmission lines are not needed. More importantly, a large portfolio of small solar systems will always work. Like a car engine, gas turbines sometimes fail to start.

Nevertheless, gas turbines are the electricity market’s standard response to peak demand because they are the cheapest type of conventional generator to build. Comparing distributed solar to the cheapest generator should understate its value as a provider of peak capacity.

It is of course necessary to correct for a few things; solar doesn’t produce at its full rated capacity in the afternoon with the sun striking at an angle and gas turbines suffer transmission line losses and output reductions in hot weather.

After crunching all the numbers, the ability of distributed solar to supply peaks has capacity value equivalent to 10-20% of the unsubsidised installation cost. That’s on top of the energy value as calculated by all the regulators. It’s worth more in the south, because of the longer days. It’s worth another 3-6% of the system cost if the system faces north-west or west.

Importantly, the value is a capital value, not cents per kWh. Capacity isn’t about how much power is produced. It’s about how much is there when you need it most.

A working paper with more detail is available here

Sign in to Favourite

Join the conversation

37 Comments sorted by

  1. Mike Stasse

    retired energy consultant

    I monitor everything in this house..... energy in, and energy out. I have my trusty energy monitors right here next to me on my desk! At 6:45 this morning, the inverters cut in (yes, we have two arrays - 1.3kW + 2.2kW) and started producing 101W. That's MORE than we need as a continuous power supply because we use less than 2kWhrs/day...... It's 7:30 now... producing 722W! And already making 32c an hour!

    report
  2. John Newlands

    tree changer

    From memory Australia's 2012 electricity production was 254 Twh of which 3 Twh came from solar. An earlier Conversation article suggested about 75% of that energy was used in the home the rest exported to the grid. With the feed-in tariff dropping to 8c per kwh Australia wide we may never get to 2m solar roofs. We are not going to be dynamiting Hazelwood just yet.

    I'm not sure about the just when you need it claim. Some hot nights can be 35C long after after sunset. I also understand in daytime PV loses efficiency above 26C. Think 45C in typical future summers. There is also the problem of extended low light conditions with August 2013 in Tasmania being a good example. People still need TVs, coffee makers and toasters when it may be several days since the last blue sky.

    Therefore PV is a helper or niche product not an energy revolution. We shouldn't take our eye off the ball of reducing coal burning for which the answer lies elsewhere.

    report
    1. Mike Stasse

      retired energy consultant

      In reply to John Newlands

      The key is energy efficiency.... even on a cloudy day, our PVs produce 10% of their rated output. That's 3.5 times our "baseload" demand, continuous.

      Nobody "needs" "TVs, coffee makers and toasters"........ they WANT them!

      One day, when the oil runs out because we can no longer afford to extract the low energy yield stuff left in the ground, we will need to live more simply so we may simply live.

      Our toaster died recently. Haven't replaced it yet. If we feel the need for toast, we can always slap a piece or two on the AGA's hotplate.... under which I bake my bread by the way!

      The TV only uses 40W, so we can still afford to watch it on cloudy days..... and coffee? Well I use a stove top Italian style caffetiere.. those coffee makers are such a wank....

      report
    2. John Davidson

      Retired engineer

      In reply to John Newlands

      If you double the difference between inside and outside temperature the power required to maintain this difference will be four times higher. For example, if you were trying to keep a house at 25 deg C you need about 2.25 times more power during the middle of a heat wave with a midday temperature of 40 deg C compared with 35 deg later in the day.
      Rooftop solar really does make a dramatic difference to an air conditioned houses external demand.

      report
    3. John Davidson

      Retired engineer

      In reply to John Barker

      John B: The amount of heat flowing into the house is proportional to the difference between inside and outside temperature. In addition, the amount of energy required to pump one unit of energy into or out of the house is also proportional to the temperature difference.
      When you take both of these factors into account the heat pump energy consumption is proportional to the temperature difference squared, i.e., doubling the difference between inside and outside temperature will increase average heat pump power draw by 4.

      report
    4. Felix MacNeill

      Environmental Manager

      In reply to John Newlands

      John, if you check out insulation and timers on air-conditioners, you'd realise it's perfectly possible to use the peaking PV power throughout the afternoon to get your house nice and cool by the time you come home from work in the evening. If your insulation is any good the place will hardly be overheated by the morning.

      And, as Mike points out below, there's a big difference between 'need' and 'want'.

      report
    5. Doug Hutcheson

      Poet

      In reply to John Newlands

      "People still need TVs, coffee makers and toasters" - or people THINK they need their gadgets. It is only because we have been sold the utility of electrical appliances that we have our current problem. Most of us will not die if our toaster refuses to toast, but we conflate 'availability' with 'need'. If we lived our lives consuming only what we truly need, life would be simpler (but don't ask me to do without my Internet, though ...)

      report
    6. John Barker
      John Barker is a Friend of The Conversation.

      Adjunct Professor at Murdoch University

      In reply to John Davidson

      John- although that Carnot formula is correct, the actual reduction of performance of airconditioners and heat pumps is not so marked. I dug around for a while to find some real COP's- it seems like the reduction of COP between 25 and 40°C would be about 25%.

      I think that some real research needs to be done, taking into account the time-of-day tariff structures as well as the thermal mass and solar PV performance. At the moment we are making best guesstimates.

      report
    7. Stan Hlegeris

      logged in via Facebook

      In reply to Mike Stasse

      Mike--

      I have to query your comment about PV performance on cloudy days.

      Call the instantaneous output from my system on a sparkling clear blue day 100%. My observation is that on a cloudy day the output drops to 40-50%. When it's raining lightly the output is still up around 10-15%. We need absolutely black clouds and heavy rain to push the daytime output below 5%, and I've never seen it lower than that. Such dark daytime conditions are highly localised, never occurring for long in any one place or over a large area.

      If your system drops to 10% on a cloudy day I suspect you have a technical problem.

      report
    8. Mike Stasse

      retired energy consultant

      In reply to Stan Hlegeris

      Yes... it's called clouds.

      And not all clouds are created equal.... ever seen clouds drop 300mm of rain in two hours....????

      report
  3. Douglas Furby

    Business Manager at music

    The proposition that South Australia is favoured by its lower latitude is challenged by the fact that Adelaide (34.93) and Sydney (33.52) are sensibly on the same latitude.

    report
    1. David Osmond
      David Osmond is a Friend of The Conversation.

      Wind Engineer

      In reply to Douglas Furby

      it may be explained better by the fact that on Jan 1, sunset in Adelaide is about 24 minutes later than sunset in Sydney. So I guess it's a combination of both the latitude, and the location's longitude relative to its time-zone.

      Craig Miller makes a good point about daylight saving time. It can certainly help solar reduce peak demand.

      report
    2. Mike Swinbourne

      logged in via Facebook

      In reply to Douglas Furby

      Although they are essentially the same latitude, Adelaide is more favouable for solar because it has fewer cloudy days than Sydney during summer.

      report
  4. Craig Miller

    Scientist

    I wonder if the uptake in solar PV in Queensland will be the factor that shifts our recalcitrant State into daylight saving time. Given that it gets light at 4ish in the summer and starts getting dark by 6pm - solar could be meeting some of the peak evening demand if we went to DST.

    report
  5. Georg Antony

    analyst

    It is good to see PVOutput used.

    It is a tribute to the enthusiasm of 'bankstownbloke', a private project that turned into a very valuable data source for serious analysis.

    report
  6. Mike Hansen

    Mr.

    Along the same lines, we are now seeing the development of Virtual Power Stations. The term (borrowed from the IT industry) "refers to a cluster of small-scale generators that collectively act like a single large power plant, thus avoiding the fluctuations that characterize individual wind and photovoltaic power facilities."

    The VPP employs a smart grid and VPP virtualization software like "the Decentralized Energy Management System (DEMS) from Siemens. It processes weather bulletins, the latest…

    Read more
  7. John Barker
    John Barker is a Friend of The Conversation.

    Adjunct Professor at Murdoch University

    The issues of peak demand and energy storage of PV systems seems to almost have "zombie" status- an issue with memes that simply won't die despite the amount of data to the contrary.

    Simply, the reduced cost of PV electricity- less than 10cents/kwh at the plug- and still going down at about 20%/year- is opening up new ways of thinking about electricity use. Storage does not have to be in the form of a battery- the house itself can be used to store warmth or coolth generated by the PVs - particularly…

    Read more
    1. George Michaelson

      Person

      In reply to John Barker

      Although a heating engineer or architect would doubtless agree with you, I think people expect that pre-cooled or pre-heated is not the same as 'the warm rush of air' if you turn the bugger on when you walk in.

      I have experienced this trying to convince older people with full aircon to soak the house down and keep it there. They preferred to keep the ambient apparent air temperature warm overnight, and then chill down to icey cold first thing afternoon. They basically fought the thermal store of the building all the time. Even suggesting they find a 2 or 3 degree sweetspot and hold it there, with a thicker doona or less jerseys later on didn't work. "thats not why we fitted the aircon"

      report
    2. George Michaelson

      Person

      In reply to John Barker

      I agree the economic signalling will change the landscape, but I worry the social dynamics of smart metering have already been hijacked. Partly by a very odd anti-RF crowd who seem to think this is one of the seven signs of the apocalypse. But also, by some reasonably well targetted questions about who actually benefits from the deployment.

      Because, smart meters appear to come with a huge logistical shift to time-of-use and the current TOU billing model is really quite different to the classic two-tarrif model a lot of people are used to.

      Without some better internal controls on electricity consumption, I fear that the TOU and smart meter will consist of a billing shift, and a buzzing red light in a hard to reach cabinet, and not a useful signal in short term cycles we can use to decide to load shift.

      report
    3. Felix MacNeill

      Environmental Manager

      In reply to George Michaelson

      Fair point - smart meters, I would have thought, are a necessary but not sufficient technology for efficient houseold energy management - we need things as simple as timer switches through to sophisticated load management systems at higher levels. Once you start to assemble the full set, things look far more promising.

      Then again, I don't have a smart meter in my house so maybe I'm not picking up the signals beamed from the mothership...

      report
    4. R. Ambrose Raven

      none

      In reply to John Barker

      Oi! Just a minnit, mate.

      This childlike faith in market forces simply erases all the bits that don't fit - tenants, for instance, who won't have solar panels because they and not the owner would benefit from installation.

      report
  8. Keijo Musto

    CEO Engineering Co'

    Nice article Kerry, but you have left out one very important area.
    That is, the availability of distributed energy production, heat and electricity, from natural gas by devices such as fuel cells.
    For detailed info' look up "Ceramic Fuel Cells" and become familiar with their potential to produce houshold power locally, on demand, at any time, at a fraction of the greenhouse emissions of grid power.

    report
    1. Mike Stasse

      retired energy consultant

      In reply to Keijo Musto

      As it happens, I know someone who runs a University faculty on sustainable energy, and she has a PhD in fuel cells.......

      She reckons fuel cells will NEVER have any impact in energy production, anywhere........

      report
    2. Keijo Musto

      CEO Engineering Co'

      In reply to Mike Stasse

      Hi Mike,

      They say, she reckons, I hear, etc carries no weight with me.
      Why not do some research in the field and then answer in your own right?

      report
    3. Mike Stasse

      retired energy consultant

      In reply to Mike Stasse

      Oh and she's actually INVENTED Ceramic deposition technology t manufacture fuel cells.....

      report
    4. Keijo Musto

      CEO Engineering Co'

      In reply to Mike Stasse

      Thanks Mike, I will read up on her papers.
      I'm not suggesting that the fuel cells are here and now and are the answer to everything, but let's not wipe them out of the equation at Susan's say so.

      In the meantime why not read up on:
      http://www.bluegen.info/

      report
    5. John Davidson

      Retired engineer

      In reply to Keijo Musto

      Fuel cells will still generate greenhouse gases and require a source of fuel. There is no place in the future for fossil fuels.

      report
    6. Keijo Musto

      CEO Engineering Co'

      In reply to John Davidson

      I agree completely John, but until renewables can supply 100% of our energy needs, I'd rather not make up the difference by using grid power derived from burning brown coal.

      report
  9. John Davidson

    Retired engineer

    Our household power bill averages about $3.00.day Smart meters sound very good but who is going to hover over a smart meter to save a few cents per day. I guess you can pay more for a power management system but it probably makes more sense to spend the money on energy saving or solar panels.
    It would also make sense if there were more controlled power options.
    There are not a lot of things in a house that really need on demand power as long as the extent and frequency of power of times is appropriate for the item depending on controlled power.

    report