As one of the world’s largest polluters, the United States has a lot to do to address the challenges of climate change. But how much can it really achieve, and how should it reach the 60%-80% emissions reductions which scientists say are needed? Edward Milford looks at the findings of a report recently commissioned by the American Solar Energy Society to investigate these questions.‘We have nothing to fear except fear itself,’ Franklin Roosevelt memorably told the American people in his inaugural address in Washington on 4 March 1933. When contemplating the scale of the challenge that climate change presents, it is very easy to sink into a mood of deep pessimism and fearfulness. After all, the physics is quite stark. The earth’s atmosphere, a thin blanket around the planet’s surface that is essential for all living systems, is just a few tens of kilometres thick (compared with the earth’s radius of 6378 km at the equator, it is roughly one 200th as thick). This fragile layer which buffers the carbon cycle has to accommodate all of the additional CO2 that is emitted by human activity. Just how much extra CO2 can it carry?
The estimates for wind power in the study were relatively conservative at 20% of total electricity demand. Improvements in grid and storage technology could improve this SUZLON
The links between human emissions of CO2, rising atmospheric concentrations of CO2 and rising temperatures are now accepted by the overwhelming majority of scientists. To stabilize global temperatures at no more than 1°C above the level in the year 2000 means limiting atmospheric CO2 concentration to 450 ppm – 500 ppm. We need to do this not just with current levels of economic activity but also allowing for a higher level of economic output that is capable of providing an acceptable standard of living for people all over the world.
The consensus is that for developed countries this means that emissions need to be reduced by 60%-80% below today’s values by the middle of the century if the climate is to have a chance of being stabilized. If we fail to achieve this, the prospects for future generations are bleak indeed. It is a daunting prospect and can very easily lead to a counsel of despair.Wedges
The problem is relatively easy to state. Are there solutions? How can we tackle this issue, and what chance have we got of achieving the reductions we are aiming for? As with any complex problem, it often helps to try and break it down into a series of smaller problems. Perhaps the best known approach to tackling climate change in this way was a paper by Stephen Pacala and Robert Socolow in an issue of Science in 2004. They adopted a large-scale, top-down approach to the problem of reducing emissions to break it into what they believed were achievable steps on the way. While there are, of course, a number of assumptions and simplifications built into their basic calculations, these do not fundamentally alter the orders of magnitude. Essentially, to level out at 500 ppm of CO2 in the atmosphere they assumed the world would need to stabilize emissions at the current value of some 7 billion tonnes of Carbon per year for 50 years and then reduce them substantially.
To achieve this would involve the displacement of 175 billion tonnes (or gigatonnes, Gt) of Carbon over the next 50 years. Pacala and Socolow divided this into seven ‘wedges’ of 25 GtC and identified what they called a ‘portfolio’ of seven possible technologies to save these amounts; none of these technologies was capable of achieving the entire saving needed on its own, but together, the authors concluded, they are capable of making the necessary savings. Each is a proven concept and either commercially available, or thought to be near commercial viability.
One of the biggest contributions the US could make would be to improve vehicle efficiency standards, particularly of SUVs
Examples of the wedges would be two million 1 MW wind turbines displacing coal power; or two billion personal vehicles achieving 60 miles per US gallon (26 km per litre) instead of just 30; or capturing and storing the carbon produced from 800 large modern coal plants.
The great advantage of this analysis is, in the authors’ own words, to ‘decompose a heroic challenge into a limited set of monumental tasks’. Thus, for instance, two million 1 MW turbines is a scale-up of a factor of roughly 50. While some wedges may be able to achieve their targeted wedge of savings, others may struggle to achieve the wedge.
Pacala and Socolow were not advocating particular technologies, and all of them have significant issues associated involved with their wider uptake. However, they were issuing a challenge if you ruled out the technology from one wedge, you had to replace it with another technology with the same potential.The ASES response
The American Solar Energy Society (ASES) decided to rise to the challenge and see whether the US could realistically meet the carbon reduction target. Throughout its 2006 annual conference in Denver, a special track of nine presentations described how energy efficiency and renewable energy could mitigate climate change. In new publication Tackling Climate Change in the US – potential carbon emissions reductions from energy efficiency and renewable energy by 2030, edited by Charles Kutscher, the results of these deliberations and subsequent analysis have been brought together.
In responding to the challenge, ASES decided to start from an assessment of the carbon reduction potential of energy efficiency and each technology and then look to see how big the wedge could be. As with all such studies, there have to be some simplifications and assumptions made, for instance in the type of generation that might be displaced (typical US coal plants emit 260 tonnes/GWh, while the overall US average is 160 tonnes/GWh).
The conclusions were very encouraging. The study’s authors were able to identify potential carbon reductions of 1200 MtC/year from the US. This would put the US on target to achieve the necessary carbon reductions by mid-century. Of this, some 57% is from energy efficiency measures and some 43% from renewables. Following this strategy would see the US generating some 40% of the electrical energy need projected for 2030 by the Energy Information Administration (EIA) – or around 50% of the US electrical need, taking into account the energy-efficiency measures proposed.What are the options?
So what are the analyses, and how radical are the proposals? Are they achievable or are they simply the unrealistic dreams of idealists? It was natural to look at the demand side first, and the energy-efficiency analysis was carried out by Joel Swisher of the Rocky Mountain Institute. Of the energy savings identified, about 40% came from buildings, and 30% each from vehicles and industry.
The strategies examined for improving energy efficiency in buildings included efficient heating, cooling and lighting appliances, control systems that minimize loads and use passive solar heat and daylight where possible, and more energy-efficiency building shells.Buildings
The detailed analysis of technologies in buildings was done by Brown, Stovall and Hughes from the Oak Ridge National Laboratory. They considered the potential from each part of a building. For roofs, they looked particularly at pigmented roofing products which reflect more of the incidental thermal energy and can show significant cooling energy savings, and at ‘smart’ roofing materials where the material will absorb heat on cold days and reflect it on warm ones. Wall systems that reduce the amount of framing generally have better insulating properties, and while these only really offer savings opportunities in new buildings, insulated sheathings could be more widely used in retrofits. Many existing windows have much better energy properties than standard ones and could be more widely used, and low-E coatings which reduce the flow of infra-red energy may also be able to offer savings. Another window technology under development is electrochromic windows with a dynamic control of spectral properties; these could save up to 40% of the HVAC energy in arid climates. Other measures include better balance of moisture-tolerant materials to reduce the need for air infiltration, use of more thermal storage and massive construction materials to buffer temperature swings, and more use of both vacuum insulation and recycled materials (with their saving of embodied energy).
Although only an emerging commercial technology, parabolic trough concentrating solar power could account for up to 80 GW of new capacity by 2030 SCHOTT
As well as the building fabric itself, there are many opportunities for savings in the energy use within the building. In particular, there are many technical opportunities to save energy on HVAC systems. These include smarter controls, particularly in conjunction with thermal storage to help control relative humidity, variable-speed air handlers and ground-source heat pumps. Better sizing of HVAC systems can also make a significant difference, particularly as the ‘rules of thumb’ often used for sizing were developed for more energy-intensive buildings.
There are significant efficiency improvements now available for oil-furnace heating systems. Water heaters can benefit from a heat pump water heater, a water heating dehumidifier, using waste heat to help heat the water and, of course, wider uptake of solar water heaters.
With the wider availability and falling costs of domestic photovoltaics, these systems will become increasingly common, particularly as they become integrated into other building components. PV can provide a valuable contribution to peak shaving, particularly where peak electricity demands are associated with cooling loads.
Overall, as early as 2010, advances in building envelopes, equipment and integration could lead to 50% reductions in the energy demands of new buildings, even when compared with those constructed as recently as 2000, and with a small or non-existent increase in capital cost. If combined then with on-site power, notably integrated PV, the possibility of ‘net-zero-energy’ buildings that are cost-competitive may be achievable by 2020.
Many of the same energy-saving options apply to large industrial and commercial buildings, though in addition, these have more opportunities to deploy intelligent building controls. Lighting is another area where significant savings could be made in larger buildings; wider deployment of compact fluorescent lights is one technique, but the use of hybrid lighting (where the visible portion of the solar energy is transmitted into the building through light-conducting optical cables) could also make significant energy savings and offers architects and designers, particularly of retrofit, significant new lighting options. Solid-state lighting also shows significant promise, with LEDs already being deployed in traffic lights and exit signs. LEDs appear ripe for further technological development, which would also open up a much wider range of applications for them.
Another option open particularly to larger industrial and commercial buildings is more use of distributed energy, typically on-site cogeneration (combined heat and power) or trigeneration (combined heat, power and cooling) systems. Such systems offer much higher overall efficiencies than the centrally-generated electricity they displace. As well as the efficiency advantages, they can offer significant further savings both by reducing the need for infrastructure development and grid extensions. For facilities that need stand-by generation anyway, they also offer better use of the invested capital cost.
The authors also identified a number of policy measures that need to be put in place alongside these technological advances. These include improved construction codes, better standards for appliance and equipment efficiency, utility-based financial incentive programmes, assistance for low-income areas and strong backing both for the Energy Star® and Federal Energy Management Programme, known as FEMP.Transport
For each US gallon (3.78 litres) of gasoline consumed in a vehicle, about 3 kg of carbon is emitted. An average US car thus emits about one tonne of carbon per year. Clearly, the simplest and by far the cheapest way to reduce this is to raise the fuel efficiency of the vehicle fleet and to remove the exemptions for sports utility vehicles (SUVs).
Using biofuels in vehicles is becoming increasingly popular. One possible move is to increase the number of vehicles that run on E85, where cellulosic ethanol is blended with gasoline. The biofuel industry is growing at a tremendous rate in the US at the moment; with recent rises in the price of oil, the use of conventional crops such as corn and soybeans to make ethanol has become a commercially-viable option. However, as prices of the feedstocks rise, the viability of these plants is affected, and the limited scale of resources means that even the estimated upper limit of ethanol production from corn would meet only 5% of current US gasoline demand.
New thermochemical processes based on gasification and pyrolysis and new biological processes, mainly enzymatic hydrolysis of cellulose, should open up the possibility of using lignocellulosic biomass (the woody parts of plants) for biofuels. This would allow agricultural residues and perennial grasses to be used as feedstocks for biofuel production. As these are still some way off full commercial viability, and some key technology targets have yet to be met, the carbon savings estimates for these technologies are more speculative than many of the others arrived at. A reasonable set of assumptions suggests that as much as 20% of the total current US consumption of gasoline could be substituted by biofuels, some 28 billion gallons per year.
There are two other significant considerations to take into account with biofuel. First, increased supply of biofuel should help to reduce volatility of oil prices as it will increase the overall available supply of transport fuels. There may also be a choice between using biomass resources for liquid fuels or for electricity production – and the carbon savings are significantly higher if the biomass is used for electricity production (particularly if the carbon can be captured and stored as well).
Photovoltaics can make a significant contribution to meeting peak demand in summer months ALTPOWER
There is another option, though, for transport – plug-in hybrid electric vehicles (PHEV). Various cities in the US have been experimenting with these on quite a large scale, notably Austin in Texas. Lilienthal and Brown from the National Renewable Energy Laboratory evaluated the contribution that could be made from PHEVs. These vehicles have a number of advantages; first, they offer the possibility of a significant, distributed energy storage resource. If charged up at times when wind speed is high but energy demand is low, they can help compensate for the intermittency of some renewable technologies. (Equally, of course, night time electricity is likely to be coal-fired baseload, and may provide a relatively high carbon footprint!)
PHEVs currently offer significant savings per mile driven. They operate with a larger battery than the current hybrid vehicles, such as the Toyota Prius, and can be plugged into a standard home, office or garage outlet. If running a low mileage per day (in common with 50% of US vehicles) they can be run almost entirely on the electric motor. Battery technology, cost and size are currently the significant limiting factors, though also the subject of intense research.
Looking further ahead, it may be that PHEVs could also return power to the grid at times. If there was sufficient capacity, this could help reduce the need for spinning reserve on the grid and effectively increase the capacity available to utilities. For the purposes of the ASES study, though, no carbon savings were assumed from PHEVs.
It is worth noting that the study makes no mention of the possible use of hydrogen fuel for transport. At the moment, the total system efficiency of such systems with the numerous energy conversions makes this an unattractive option. If in future there were to be spare renewable energy generation capacity, it might make sense to look at hydrogen as a possible energy carrier for transport systems, but at the moment, the potential for any carbon savings are not significant.
It is also important to remember that improved fuel-efficiency standards are far and away the cheapest and most effective way to cut down on CO2 emissions from the transport sector. A Hummer run on biodiesel is not an efficient strategy!Renewable energy
Having considered the savings available through efficiency measures and in the transport sector, the report then looks at the contribution that each of the main renewable energy technologies can make to the energy supply.Concentrating solar power (CSP)
Mehos and Kearney carried out the analysis of the potential for CSP. The technology requires direct-normal solar radiation (rather than using any part of the scattered or diffuse radiation). This limits the sites suitable for installation to areas with over 6.57 kWh/m2/day. Nevertheless, the Southwest of the US has a very significant area that could potentially be used for CSP, which typically requires about 5 acres of land per megawatt installed. While the total theoretical potential for the US is over 7000 GW of capacity (about seven times the current total US electrical capacity) when distance to transmission lines and other factors are taken into account, about 200 GW of optimal locations for CSP can be identified.
CSP technology using parabolic trough systems is relatively well understood, and the key to its deployment will be the cost reductions that can be achieved by larger-scale implementation of the technology. A plan for 4 GW of CSP to be installed by 2015 is under consideration by the Western Governors Association. Assuming that this goes ahead and that the investment tax credits for CSP are extended to 2017, modelling suggests that an installed capacity between 30 GW and 80 GW of parabolic trough systems could be in place by 2030. Adding in six hours of thermal storage at each plant increases the capacity factor to 43%, and gives a carbon saving between 48 MtC/year and 79 MtC/year by 2030, with an average of 63 MtC/year.
There are other means of using CSP to produce electricity, such as two-axis tracking parabolic dishes with Stirling engines, and power towers with tracking heliostats. However, these are still some way from achieving the track record in deployment of parabolic trough systems, and were not considered as part of the study.Photovoltaics
It is particularly difficult to estimate the likely impact that solar photovoltaics (PV) could realistically have on carbon savings. It is a distributed technology, with the overall contribution likely to be made up of a lot of small installations, some of which will have good characteristics, many of which may have rather poor capacity factors. The PV industry is growing fast, but most module manufacturers are operating at full capacity, and increasing the output will require investment in every step of the process. However, the German PV industry (see Renewable Energy World, March-April 2007, pp 44-52) has shown what can be achieved if the market conditions are favourable. If the PV industry in the US crosses an equivalent tipping point, it is possible that some of the numbers presented in the ASES study will seem very conservative.
Geothermal energy reserves are hard to quantify, although it was estimated that they could save the emission of up to 100 Gt of carbon in the US each year © WWF-CANON / PETER PROKOSCH
Denholm, Margolis and Zwiebel carried out the study to try and quantify the carbon reductions available from PV. They assumed that deployment would be predominantly on rooftops across the US, and with due allowance for shading and orientation, estimate that there are between 6 and 10 billion m2 that could be usable. With today’s typical PV systems achieving about 100 W per square meter, and an average capacity factor of 17%, PV installed across this area could generate between 25%-40% of current US electricity consumption.
It has been estimated that some 10% of the US electricity grid energy, or 275 GW by 2030 using the EIA figure after efficiency adjustment, could be supplied by PV without creating grid management issues. The US PV industry’s own road map aims at a target of 200 GW installed by 2030, and this is the figure that was used to calculate potential carbon savings. This then gives a carbon saving in the range of 48-78 MtC/year, an average of 63 MtC per year, coincidentally the same as the figure for CSP. It is worth noting that this does represent a 500-fold increase in capacity compared with that currently installed in the US; Germany achieved a near 50-fold increase in installed capacity between 2000 and 2006.
There are, of course, other factors that could improve this figure. Tracking systems can be used to increase capacity factors. There could be significant potential for ground-mounted systems as well as the roof-mounted ones considered in the study, as has also been seen in Germany, depending on the incentives available and the comparative cost of generation. Other technological developments such as thin-film and concentrating PV could enhance the performance.Wind power
The wind power study was carried out by Michael Milligan from NREL. Installed wind energy capacity in the US has been growing fast, with over 11,600 MW installed at the end of 2006, of which over 4800 MW has been installed in the last 2 years. The cost per MW installed for wind is likely to continue falling, and this, together with incentive programmes and tax credits, will largely determine the uptake of wind energy in future.
Key assumptions in calculating the carbon saving available through greater deployment of wind energy in the US were that the tax credit would continue at the current rate of 1.9 cents per kWh to 2010, then be phased out linearly until the year 2030, and offshore wind was not considered, despite its rapid uptake in Europe. The total capacity was also limited to 20% of the electrical energy demand, 245 GW, to ensure dispatchability. Together, this is quite a conservative set of assumptions, and the 20% target seems quite reachable.
It is also important to consider the location of wind power plants in more detail and how much of the existing transmission grid is available to transmit wind-generated electricity from areas where it is abundant to areas with electricity demand. Even building in additional transmission and limiting the regional supply of wind energy shows that wind energy, unlike PV for instance, can achieve rapid market penetration in the near term due to its competitive costs. It is likely that this would level off as the best sites get used and dispatchability considerations further limit the use of wind energy.
Taking all this into account, the carbon savings from wind energy are impressive; they vary from 138 MtC/year to 223 MtC per year, with a mean value of 181 MtC/year.Biomass energy
This study was carried out by Overend and Milbrandt from NREL. It takes as a starting point a study carried out by Oak Ridge National Laboratory, which estimates a potential 2025 crop and biomass residue contribution of 1.26 Gt. A further assumption is from a study for the Western Governors Association that showed in the 18 western states the 170 Mt of dry material likely to be available was in theory capable of generating 32 GW. In practice, only about 50% of this would be economic at $80/MWh (or 8 cents per kWh). Using this as a national average suggests that although the 1.25 Gt is capable of generating some 235 GW, only 110 GW of this would be economically available. This is a 10-fold increase over the current biomass electricity capacity.
The study assumed the lowest-cost plant option; for larger plants above 15 MWe, this is usually integrated gasification combined cycle (IGCC). Below 15 MWe, this is usually either a stoker with steam turbine or a combined gasifier and internal combustion engine. The calculated carbon savings are in the range 139-225 MtC/year with an average of 183 MtC/year. Note that the resource estimate was for 2025, so using this for 2030 should be conservative, and as biomass can be used for base load and thus compete more easily against coal-fired generation, the carbon savings may be closer to the higher figure.
There are other opportunities for carbon reduction using biomass. First, some carbon is sequestered into soil through the process of growing crops, and changes in agricultural practice, such as moving to no-till systems, could improve this carbon fixation. Similarly, it might be possible to sequester and store some of the carbon from pyrolysis by returning char residues to otherwise infertile soilsGeothermal
The total geothermal potential is hard to estimate as there are a number of potential geothermal resources that are currently not tapped today. Of the current peak production of about 2200 MW, almost all uses hydrothermal resources, naturally occurring hot water or steam that is within a kilometre or so of the earth’s surface. Power is generated either by using the water to drive a Rankine steam cycle or, if the temperature is lower, using a fluid with a lower boiling point than water in what is called a binary cycle.
In contrast to wind and solar energy, where access to the resource is easy but conversion may be less well understood, the challenge with geothermal is access to the resource. The energy stored at depths of between 3 and 10 km is vast, plenty to meet existing demands for hundreds of thousands of years, so even being able to extract only a small fraction of this economically could displace a significant fraction of fossil fuel demand.
The paper by Vorum and Tester on geothermal resources does consider three different ways to use some of these other geothermal resources. First, ‘enhanced geothermal systems’, or EGS, use water injection under pressure to extract heat, and this can be used to expand the capacity of existing hydrothermal reservoirs. This can also possibly be used with much deeper, hot dry rock in ‘basement EGS’. Thirdly, depleted oil and gas wells may also offer a relatively easy source of accessible heat.
Stimulation for EGS is conceptually and mechanically simple and has been used in oil and gas production for many years. However, it is not yet proven in geothermal systems at commercial flow rates and heat recoveries. Given these uncertainties (both the scale of the resource and the development needed to harness more of it), the figure for carbon saving from geothermal is more uncertain than many of the others. The authors estimated a total of 100 GW of capacity from the four different sources, resulting in savings between 63 MtC/year and 103 MtC/year. The mid-range value was therefore 83 MtC/year, though as with biomass, geothermal is a base load power and thus competes against coal-fired power plants.Summary
The total carbon savings identified for 2030 are roughly between 1000 and 1400 MtC/year for the US, with a mid-range value of 1200 MtC/year. This would be on target to achieve reductions of 60%-80% from today’s figure by 2050. Roughly speaking, the energy-efficiency measures identified allow US carbon emissions to remain more or less level, the deployment of renewable energy technologies then provides the reductions.
The contributions outlined would result in renewable energy contributing just over 50% of the total grid electricity in 2030. An additional consideration would be what the mix is at different times and the impact this might have on base-load power production.
It is also worth noting that there were some significant limiting assumptions made. Wind was limited to 20% of grid electric generation with no off-shore generation. PV was limited by production capacity and only to roof-mounted systems. CSP assumed a carbon value of $35 per tonne of CO2. Other contributions were not considered. In particular, solar industrial process heat and other solar heating and cooling systems. In addition, improvements in electrical storage and more efficient transmission lines were not considered, both of which would increase the potential carbon saving. Nor was any form of ocean energy taken into account – this is still too far off commercial applicability to make a meaningful estimate of its carbon saving potential, though research continues apace and various prototypes are being tested at sea.
The full paper is available as a download from the ASES web site. It runs to over 200 pages and contains a lot more detail of the calculations and the assumptions than is possible to begin to include in a brief summary such as this. It is careful, comprehensive and relatively cautious – some of the more bullish advocates of each of the technologies would claim that the potentials are underestimated.
There are two wider messages I hope are received loud and clear. The first is the overwhelming case for moving ahead rapidly, now. The potential from efficiency and renewables is enormous. They are well understood, modular technologies and are relatively popular with the public. Rolling them out on a wider scale is simply a management and economic issue and does not rely on significant technology breakthroughs or novel engineering solutions. Furthermore, these technologies mainly use domestic resources and do not rely on foreign imports.
The second message is that if governments do decide to throw huge sums of money at the energy issue, renewables and energy efficiency are still the answer. The ASES study assumes today’s economic parameters and does not rely on massive government hand outs and huge subsidies, just some pragmatic tax credit extensions. The background economic climate is only likely to get more favourable for these renewable technologies, not less, as limited resource availability will inevitably push fossil fuel prices higher in the long run. If there are to be billions in subsidies, they will go a lot further and achieve a lot more if they are invested in the solutions outlined in the ASES study rather than elsewhere.
As for those fears, there is good reason to put the despair aside – there are solutions. The fear should instead be replaced by a steely determination to ensure that it is the best solutions that are adopted.
Edward Milford is Publisher Emeritus of Renewable Energy World and Chairman of publishers James & James/Earthscan