Does the implementation of a small scale grid tied photovoltaic array have any effect on fuel consumption at the power plant and thus on GHG emissions?
Would implementation of grid tied photovoltaic/battery based systems make more sense if reduction of carbon is the primary goal?
William vonBrethorst is the president of Planetary Systems, a design and installation firm specializing in battery based renewable energy systems. He also has over 20 years experience as a project engineer in the power generation field. This paper investigates his ideas regarding integration into the grid of simple PV systems compared to PV systems with battery storage. He posits,
“Simply putting some PV power back into the grid DOES NOT result in a net scale back of CO2 from the utility source...this is physically impossible ... Most power sources... are always at max output, both to maintain voltage and frequency based on potential load, and because it takes about 16 hours to bring a plant online from stand-still.” 1
Using grid tied PV coupled to a battery based storage system that is appropriately sized to supply the entire house with 80-100% of its electricity needs effectively removes the house from the grid. This method of embedded generation creates a much more significant drop in demand and has more likelihood of causing an effect at the power plant, particularly when adopted on a large scale.1
If this is true then it has implications for policy planners and micro-generation professionals regarding grid operation and the effect of intermittence, renewable energy subsidies and system design, and climate change amelioration. It also can help inform homeowners as to logical choices when considering home based RE systems and carbon footprint.
Several factors need to be examined when comparing the carbon impact of simple grid tied systems to grid tied battery based systems; the ability of the grid and power plants to respond to small changes in demand, the overall efficiency of both systems and their ability to create a significant reduction in demand and the environmental impact of added batteries compared to the increased likelihood of GHG reduction.
Current Response Ability
“... burning of coal or natural gas, and steam production are totally decoupled from the spinning turbine and the electrical output. Steam is needed constantly, at a constant pressure and must be up all the time due to the load demands which can change rapidly. There is no direct relationship between the fuel consumption for steam and the spinning turbine and its electrical output except that, if this plant is a base loading plant, there always needs to be a full steam output and pressure available. Electrical load is a relative constant to keep potential applied to the grid to maintain voltage and frequency regardless of actual load. Essentially in the case of coal, gas and nuclear plants, the fuel rate is constant.” 3- William vonBrethorst
Even in the case of very large deployment of RE, the ability of the power supply and transmission system to respond is doubtful.
“...all generators, especially large base load plants, have limits to the rate at which they can ramp up or down...there are fundamental limits to how much base-load plants can reduce output, particularly if they need to increase output a few hours later.” 4
Coal, gas and nuclear power plants have a minimum output. If
“ plant output needs to be reduced below this minimum load, it may need to be completely shut down, requiring a costly and... lengthy restart process. Given the relatively short window of high PV output, it is unlikely that these plants will be able to respond to the reduced demand.” 4 - P. Denholm, R. Margolis Solar 2006 Conference Paper NREL- 27/03/07
A reduction of fossil fuel use appears unlikely to happen at existing base load plants due to the operational restrictions just discussed. Similarly at peak load plants which are also responding to projected load the inflexibility of the current design hinders progress. GHG emissions reduction will more likely come about through massive reductions in demand causing the shut down of fossil fuel power plants altogether or through the replacement of those plants with centralized RE plants coupled with large scale storage systems.
“ A new coal fired 600MW plant may have a thermal efficiency of nearly 40%...may take 8 hours or more to reach full power and efficiency... run at part load...its thermal efficiency may...only reach 35%. It is obviously best if this kind of station is run continuously at full power. At the other extreme a small 30MW open cycle gas-turbine station may have thermal efficiency of under 30%...such a station can be run up to full power in a matter of minutes...and may only be...run for a few hundred hours a year.” 2 - Bob Everett 2003 Energy Systems and Sustainability: Power for a Sustainable Future
The effect of efficiency within the current system is debatable. Other slightly more optimistic sources indicate that coal fueled power plants can be up to 46% efficient.2 In addition the grid is operated at very high voltages and loss estimates vary widely. They are reported at 7% 5 up to 70%.3 This wide disparity is a source of concern about the data but the nuts and bolts of the system still mean that electricity arriving at a household has been through at least 5 transformers, each with an affiliated loss, not to mention the losses involved in hundreds of kilometers of cable. This has relevance to the usefulness of embedded generation.
A typical home based simple grid tied PV system outputs its maximum power at midday, off peak, usually when the owner has transferred her load to a work environment. Once the amount used in the house at any given moment is subtracted, the remaining quantity is fed into the grid through a synchronous inverter. This small output cannot even begin to overcome the high voltages in the grid. As Mr. vonBrethorst states,
“Rooftop generation from homes is absolutely useless to the grid...The small inverter CANNOT force even a few hundred watts back in to the grid in any way that benefits anyone else. The generation source is TOO SMALL... Even if the inverter ... could push voltage into the grid through the transformer, it would not be enough to keep the neighbor’s house going... Even a thousand such units would have the same problem since they can only synchronize to the grid and not each other.” 3
In addition, consumer behavior is critical. A marketing report from BP Solar states, “Most potential grid tie customers are affluent consumers who pay no attention to their electrical usage.” 6 The validity of this statement requires further research both here and in the US, as the prevailing belief seems, anecdotally, to be just the opposite. Recent research indicates a 6% improvement in conservation behaviours of homeowners who install grid tied systems. (Kierstad)
The design of an affordable grid tied battery based PV system requires appropriate efficiency efforts. In order to live off a reasonably sized bank of batteries, the usage in the house must be brought to a minimum. Mr. vonBrethorst suggests,
“Energy management is the key...We use our... systems as energy management tools... wise energy choices can reduce any home’s nominal load by about 80%. This is much different than just selling back, in that the load is reduced 24 hours a day. If a simple majority of homes did this, we would actually be able to shut down plants using old fuels because the larger base loading systems would not be needed. With grid-tie and no energy storage, they are needed because, any day that there is no sun, less wind, all energy sources are needed and especially base loading plants because this grid-tie energy simply is not available - suddenly and for quite a while”. 1
Clearly if all homes merely undertook extensive efficiency improvements it would significantly reduce demand whether or not they ever pursue renewable energy.
Small scale battery based renewable energy systems can achieve efficiencies of up to 96%.1 During the day while the owner is at work and the PV array is producing more than enough to keep the small high efficiency triple insulated 12v fridge ticking over, the excess will go into the batteries rather than into the grid where it is lost. Using the stored power at peak time, in the evening, then removes the home from the surge load on the grid. Another promising option is to direct the surplus power to plug-in hybrid vehicles thus achieving additional carbon reduction by supplanting petrol use.
Much of the environmental impact of batteries is attributable to the fossil fuel usually used to charge them; this is clearly mitigated by using renewables. Further, the impact of production is significantly mitigated by extensive recycling.7 The lead acid battery industry is mature, over 100 years old, with well developed, widespread and mandated recycling technology. The lead metal recycling rate is 98.3% with 90% recuperation of electrolyte. The remaining 10% is neutralized prior to disposal.7 Mr. vonBrethorst states that his company
“has installed about 1600 battery-based systems... all the systems we have installed ... are still in service, sometimes for over 17 years. AGM battery technology, in grid-tie applications can last over 20 years so battery longevity is not an issue even with lead-acid battery technology.” 1
Assuredly batteries have environmental impact, but it should be considered in light of the benefits of the increased likelihood of removing fossil fueled power plants.
The scope of this paper does not take into account the environmental impacts of the mining and transportation of coal or uranium, the drilling for oil and natural gas, nor the health effects of coal combustion. Neither does it address the life cycle analysis (LCA) of solar panels which have their own toxic waste issues or for that matter the lead mining industry or the production of sulphuric acid for battery electrolyte. Everything we do has impact, we must make decisions about how best to maximise the benefit from our impacts.
There is certainly debate about the issue of the effect of grid tied small scale RE on greenhouse gas emissions. I was unable find any research that could show a direct correlation between installation of grid tied small scale RE, whether battery or non battery based and reduction of greenhouse gases. However use of fossil fuels for generating electricity is increasing despite significant increases in wind and solar capacity. In the US in 2005, just under 87 percent of the 15 gW of new capacity to come on line was fossil fuel fired.8 At the same time 71,000 kW of grid connected solar PV capacity was installed.9 While this fails to prove that RE is not reducing carbon emissions, it does suggest that there needs to be a major paradigm shift in order to maximize the effect of installed RE.
Professor Mark Diesendorf of the Institute of Environmental Studies at the University of New South Wales in Sydney Australia stated in the debate on the Renewable Energy Access website,
“When RE substitutes for fossil fuel, it reduces CO2 emissions. Even PV without batteries can substitute for some intermediate and peak load fossil fuel...According to modeling by my group and others, wind power can replace base-load power stations with the same annual electricity generation.” 1
I was unable to gain access to either the modeling he mentions or any other research to back up his claims. However, in the UK, wind farms are able to compete on the power pool market which would suggest that they are replacing other types of generation. Perhaps they are replacing fossil fuel generators that have not been built as a result of their capacity, but it does not necessarily follow that existing stations are reducing fuel usage due to increased wind generation.
P. Denholm and R. Margolis at National Renewable Energy Laboratories in the US had this to say about energy storage. It
“represents the ultimate solution to the problems of intermittence. Not only could energy storage absorb excess PV generation, but it could also aid in increasing the overall flexibility of electric power systems by decreasing dependance on traditional base load generation.” 4
Anyone involved in micro-generation at any level would do well to consider techniques of energy storage as a means of improving the performance of systems and thus climate change amelioration. Should homeowners be made aware of the issues surrounding carbon impacts of installed systems? Should ecological footprints and energy performance reports assume that carbon reductions automatically occur with installation of grid tied PV or for that matter efficiency improvements? This debate raises questions about the effect of attempts to cut electrical energy use at any level. Would the political will to pursue efficiency targets and PV installation be diminished if it meant making drastic changes in lifestyle and shutting down power plants? Would increased grants and subsidies to cut fossil fuel use directly, such as solar thermal installations and public transport, be more efficacious?
Given the apparently short window of time we have to reduce GHG emissions it would seem prudent to consider ways to maximise the gains we can achieve through renewable energy installations. Efficiency with or without renewable energy has been seen as an obvious first step and made a priority based on the assumption that it is the quickest path to large scale demand reduction. Efforts at the commercial, homeowner and tenant level should include an understanding that small efforts could be delaying real reductions in emissions. If we are to pursue widespread embedded generation then schemes such as private wire networks that allow small scale generators to sync to each other as well as equally widespread use of storage technology should be installed. Such schemes would also multiply efficiency gains as the surplus power is stored or used locally thus avoiding transmission losses. We don’t need to wait for the hydrogen economy for this to happen. Battery technology is currently up to the task and whether it is utilized in the form of household battery banks, centralized flow batteries, or plug in hybrid vehicles it should be designed into systems from the household on your street to the households on Downing Street and Pennsylvania Avenue where policy documents are considered.
1 vonBrethorst, William 27/01/07 comment on Renewable Energy Access.com forum discussion “Grid-Tied or Battery-Based RE Systems?”, 25/01/07 (http://www.renewableenergyaccess.com/rea/news/podcast?id=47223#readercomments)
2 Bob Everett 2003 Energy Systems and Sustainability: Power for a Sustainable Future chapter-Electricity, 2nd edition, Oxford University Press, Oxford,in association with The Open University, Milton Keynes UK, pp 377-378
3 vonBrethorst, William (email@example.com) RE:battery-based systems email to CR Worthington (firstname.lastname@example.org), 20/03/07
4 P. Denholm, R. Margolis 04/06 Very Large Scale Deployment of Grid-Connected Solar Photovoltaics in the United States: Challenges and Opportunities Solar 2006 Conference Paper NREL- p 5, 27/03/07 http://www.nrel.gov/pv/pdfs/39683.pdf
5 US Climate Change Technology Program- Technology Options for the Near and Long Term, November 2003- Page 34, 01/05/07 http://www.climatetechnology.gov/library/2003/tech-options/tech-options-1-3-2.pdf
6 “Battery or no Battery” online fact sheet- p 35, 25/03/07 http://www.google.com/search?q=cache:jxagaxlSTwQJ:www.greenenergyohio.org/page.cfm%3FpageId%3D538+dankoff+solar+BP+Solar+battery+factsheet&hl=en&ct=clnk&cd=5&gl=uk&client=firefox-a
7 J Matheys and W. Van Autenboer 2004 SUBAT: Sustainable Batteries Work Package 5: Overall Assessment Final Public Report - A "Specific Targeted Research Project"
funded by the "European Commission"- Vrije Universiteit Brussel - ETEC, pp 4-5, 26/03/07
8 Energy Information Administration website US Coal Supply and Demand 2005 review- Consumption, 2nd paragraph, 27/03/07 http://www.eia.doe.gov/cneaf/coal/page/special/feature.html
9 International Energy Agency web-page table-Installed PV power as of the end of 2005:
in reporting IEA PVPS countries, 01/05/07 http://www.iea-pvps.org/isr/01.htm
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