by: Michael Hoexter
Climate scientists, environmentalists and some political leaders have been telling us that we should stop using coal to generate electricity or at least use it with the as yet untried carbon capture and sequestration (CCS) technology. James Hansen, one of the leading climatologists, has likened the trains laden with coal from Wyoming’s Power River Basin that roll through America as “death trains”, referring to the potential extinction of species as the climate rapidly changes. Al Gore and as well as other environmental advocates and political leaders around the world have called for a moratorium on new coal-fired generation without CCS. But where does this leave electric utilities in the US, Europe, Africa, and Asia who depend on present and planned coal-fired power plants to supply customers with power?
As it turns out, power companies and merchant generators are racing to permit new coal power plants in unprecedented numbers (there are now 121 plants under construction or in planning in the US), in part to meet soaring demand for electric power and, perhaps, in part to "create facts on the ground" before more stringent regulation is put in place.
What if a new coal plant without CCS moratorium was actually put in place? What if a substantial price was put on carbon dioxide emissions ($100/tonne of carbon dioxide=$367/tonne carbon)? Then conventional coal plants, depending on the type of coal they used would produce electricity costing somewhere in area of $.15 to $.20/kWh. If one or both of these "what ifs" become reality, there will be additional motivation for utilities to find alternatives to the use of coal as well as experiment with CCS and push for new nuclear plants.
What if regulators additionally placed a price premium on electricity produced by renewable power plants in particular ones that could perform like coal plants? If sustainability thus becomes an additional criterion, power companies would need to boost their bottom lines by focusing more on renewable coal-replacement alternatives rather than recuperating coal or nuclear.
All of the foregoing assumes that, in fact, electric utilities had not already voluntarily committed themselves to creating a sustainable alternative, leading rather than following, which some might. Such a position would confer a long-term competitive advantage to those companies who get themselves ahead of the game.
Why the Attraction to Coal?
Why do utilities like coal? For one, it's been cheap and very plentiful in areas with significant industrial economies, in particular the US, Russia, Northern Europe, China, India, Australia, and South Africa. Electricity from coal sometimes costs as little as $.01/ kWh with an already depreciated power plant with plentiful coal. In many regions of the world, utilities don't worry about running out of the primary energy to generate power, because they feel they can always get more coal and have a historical relationship with the coal mining industry (they are the industry's largest customer by far).
Furthermore, coal-fired generation technology is a known quantity. Coal-fired steam engines have been in regular use since the late 18th Century and these heat engines were retooled as electrical generators in the late 19th Century. Current technologies employ turbines and convert about 30% of the energy of the coal fire into electricity. There is on most continents an infrastructure of railways, canals or shipping that enable coal to be transported thousands of miles allowing power plants to be located nearer centers of power demand (cities and industrial areas).
Because coal is cheap, accessible, an energy store, and it takes a while to start and stop a coal fire, coal plants are used for two types of power: baseload and load-following power. Baseload power plants are on most of the time, to supply the constant baseline of power demand. Load-following power follows the daily rise and fall of power demand from the early morning to the evening. A coal plant can be set to either be constantly on at a certain level day and night or can be ramped up and down within a period of minutes. The other kind of power plants, peaker power plants for rapid spikes in power demand, require quicker responses than a coal fired plant can generate and in the contemporary power system are usually natural gas fired or hydroelectric.
To replace the functions of a coal fired power plant, a renewable power plant or ensemble of power plants would need to be economical to run and be able to run around the clock for days on end. How economical these plants would need to be would depend in part on the regulatory environment and public sentiment about the value of clean power. Additionally, the ability to ramp up production during the day would be a bonus, allowing the power plant to follow typical electricity usage.
Renewable Coal Replacement Strategies
Renewable coal replacement strategies can be divided into two categories: power plants and, what I am calling, power plant ensembles. The former are self-contained power plants that can duplicate the power output of coal plants while the latter are combinations of power plants or power plants with separate energy storage that can do the same.
Renewable Power Plants
1) Hydroelectric with year-round water supply - Hydroelectric plants can replace the function of coal-fired power if they are built in a location with sufficient water flow and vertical drop. Other environmental conditions include adequate rainfall year round or seasonal rainfall with adequate storage. The output of hydroelectric plants can fluctuate with seasonal and yearly rainfall so these plants are vulnerable to drought. In addition, dam-building unfortunately disrupts river environments so the demand for power needs to be balanced with other economic and environmental concerns. In the developed world, hydroelectric potential of larger rivers has already been exploited, but undeveloped smaller and middle sized rivers may offer opportunities for further new power generation. In the developing world there is still room for growth of hydroelectric, yet these projects will not keep up with electric demand in the most rapidly industrializing areas like China.
- Mature technology
- Accepted by power industry
- Capable of baseload, load-following and peak load (i.e. dispatchable) production.
- Smaller and midsize plants can be less disruptive of rivers and settlement, easier to permit and build.
- Room for as much as two-fold growth in the developed world with smaller and midsize plants.
- With dams represent a large energy store and are dispatchable.
- Room for as much as ten fold growth in the developing world.
- Affordable with state subsidy in dam building; dams sometimes run by government as a public good.
- Limited by geography and rainfall
- Potential insufficient to meet current worldwide power demand alone
- Vulnerable to drought, climate change, and seasonal fluctuations in water flow
- Large dam building disrupts river ecology and existing human settlement if within reservoir.
- Large dams require lead-time of years to study, permit and build
- Large dams are very capital intensive
- Competes with other economic uses of river
2) Solar thermal electric with >16 hour Thermal Storage - The most mature technology in the area of newer renewable energy alternatives, solar thermal electric taps into the direct sun of deserts and dry Mediterranean climates. Sometimes called concentrating solar power or CSP, these plants use mirrors to concentrate the sun on a thermal fluid that transports heat either directly to generate electricity in a steam generator (similar to those in coal, gas or nuclear plants) or is stored in a tank for later use to generate electricity by the same method. These tanks can use water, a thermal oil, or molten salt (a mixture of melted ionic compounds [salts] that have favorable thermal properties) as the heat transfer fluid and heat storage medium. There are now several existing power plants in the Southwestern US and Spain with more planned. A small fraction (less than 2%) of the area of the world's deserts (Sahara, Arabian, Kalahari, American Southwest, Mexican Sonoran, Atacama, Takla Makan in China, Australian Outback) would supply all of the world's energy demand with sufficient storage and transmission infrastructure. While one US government-funded demonstration power plant in the 1980's was able to produce power continuously for 7 days, current commercial designs are not explicitly designed with the excess storage and solar collectors and that allows continuous operation that would replace baseload power. Coal replacement plants will have adequate ratio of collectors to storage to turbine power rating throughout the year. During the winter solstice, all of these plants will produce as much as 40% less power than during the summer solstice, so design of a coal replacement plant will need to account for winter demand, or supplement winter production with other sources.
- Rapidly maturing technology
- Can re-use parts of power engineering knowledge base in turbine and power plant design
- Intuitively understandable by lay people as a source of power (desert=sun=heat=steam=electricity)
- Online for years in the American Southwest/demonstration of always-on capability in experimental setting
- Plants could theoretically could exceed 85% capacity factor with sufficient sun, collector-to-turbine-power-rating ratio, and heat storage
- Potential to meet all of world energy demand with low impact footprint
- Solar energy approximately follows daily electric load with a few hours offset and can operate as a peaker plant with storage.
- Theoretically plants could be built within a period of one to two years
- Currently affordable for developed nations willing to pay premium for clean power
- Currently more expensive than depreciated coal plants inclusive of transmission costs
- Footprints of plants (though in remote locations) are larger than conventional power plants
- Overlooked and underfunded for a couple decades.
- Requires trans-regional cooperation between electricity retailers or national energy agencies to reach full potential
- Requires building of new transmission infrastructure including long distance lines reaching areas distant from deserts.
- Alternative power plant cooling that minimizes water use reduces plant efficiency
3) Enhanced Geothermal Systems (EGS) - Geothermal power can be divided approximately into two categories: traditional hydrothermal and EGS. Traditional hydrothermal uses natural steam and heat reservoirs and flow close to the surface but is only applicable in certain locations (parts of Western US, Iceland, Italy, Hawaii). EGS creates engineered geothermal wells by drilling deep in earth's crust and creating a water flow circuit that collects and transports the deep heat to the surface and could create geothermal power plants in most areas of the globe. EGS is still in experimental stages but studies of heat content and flow indicate that it can eventually produce more than enough power for the world. Geothermal and EGS can produce baseload power as the flow of heat to the surface remains at a constant level, day and night once a well has been properly calibrated. Problems remain with engineering functional water flow channels at depths of 4 km and deeper with the goal of 10 km allowing for widespread commercialization. EGS will build on existing deep drilling technology from petroleum exploration and competes directly with oil exploration for drill rigs.
- Power could be produced close to load centers
- Builds on existing geothermal expertise
- Builds on existing drilling expertise
- Hydrothermal geothermal is a proven source of baseload power for over 100 years
- Geothermal plants have a high capacity factor (>85%)
- Potential to meet all world wide energy demand once fully developed
- Provides a post-carbon role for petroleum industry
- Still in experimental stages
- Competes with oil exploration for drilling rigs
- EGS wells and techniques need to be tailored to local geology
- May not scale up as rapidly as demand for carbon-free power
- No current existing EGS power plants online
- In most locations, heat extraction over a period of decades will deplete stored heat at a rate that requires a couple centuries of replenishment to restore original rock temperature: power plants may need to be moved after several decades.
4) High Altitude Wind - More experimental than the foregoing, the winds in and around the jet streams of the northern and southern Hemispheres located between 30 and 60 degrees latitude at heights from 4 to 11 km have some of the highest power density of any renewable energy sources (over 10 kW/m^2 at 10km altitude). The winds at some of these altitudes blow consistently at varying speeds with some north-south movement of the jet streams themselves. Advocates of high altitude wind speak of capacity factors for wind turbines of varying designs in the range of 60 to 95% depending on location. Various designs have been proposed, some that use kites, some that use dirigibles, some carousels. Technical challenges include weatherproofing, lofting and tethering these turbines with light, high capacity conductors. Permitting and liability concerns may develop if these aerial plants are suspended over land and follow the stream as it wanders. Coordination with air traffic is also a must. Early experiments and prototypes might use the lower altitude high-speed winds of the Southern Hemisphere that at the surface of the Southern Ocean make up the "Roaring Forties".
High power density of winds lead to low potential costs (fewer power plants per unit area with more power)
- Located at mid-latitudes where there are high population/ energy demand centers
- Wind speeds have been continually documented by atmospheric research
- If windspeeds are consistently above a certain minimum relative to a turbine's power rating, can function as baseload power
- Has the potential to meet all power demand
- Still in experimental stages; a new technology
- Energy production limited to latitudes greater than 30 degrees
- Many competing and divergent designs
- Field testing will require airspace permits
- Lightweight high capacity conductors/tethers have yet to be invented
- Conductors and mid-altitude turbines may need to resist lightning, violent weather.
- Wind does not have constant velocity might not reliably function as baseload power
- Highest velocity wind is at 35,000 ft./ 10 kilometers in height
- Jetstream designs may compete with aviation for airspace
- Exact ground footprint unknown
5) Ocean Thermal Energy Conversion (OTEC) - Uses differences in temperature of top layers of tropical oceans with the cold of the lower depths to generate electricity using a heat engine/turbine. Applicable in tropics where solar and wind energy flux is inconsistent or weak. OTEC is still in experimental stages. Extraction of volumes of cold water has unknown effects on geophysics of deeper cold layers of the ocean. Has not been the subject of a consistent level of funding. Cold water can be used to promote agriculture and supply air conditioning on tropical islands.
- An energy source for tropical areas without consistent wind or sun.
- Can function as baseload power
- Cold water has additional economic uses in tropics
- Can replace expensive diesel power generation on tropical islands
- Still in experimental or early prototype stages
- Commercial costs are unknown
- Underfunded for many years
- May have unknown effects on ocean environment including risk release of methane hydrates (strong GHGs) from deep ocean.
- Rate of recovery of cold layers of ocean unknown
Power Plant Ensembles
Like a musical ensemble, an ensemble of power plants can coordinate across as many as thousands of miles if they are on the same transmission network and shape their output to create a joint output profile.
1) Continental Supergrid for Wind - Proposed by wind advocates in Europe and the US and the large utility American Electric Power in the US to balance wind variation across space. Foreseen as connecting conventional large wind turbines either offshore on onshore. Requires major transmission infrastructure and sophisticated grid management systems.
- Uses mature technologies
- Wind in favorable locations produces electricity at competitive costs
- Taps into known resources
- Distributes energy development and production through a number of regions
- Has support of existing players in renewable energy and transmission
- Has the potential to cover much or all of electric demand in the US and Western and Northern Europe
- It is unknown whether combined wind energy in a geographical area can at all times cover electric demand; may require additional technologies to firm.
- Requires complex cross-regional and cross-national coordination.
- Transmission is expensive.
- Selection of type of transmission (HVAC vs HVDC, above ground, underground) factors in a variety of political, technological and economic factors.
- Not applicable as applicable where wind is scarce.
2) Regional or Continental Linkage of Wind with Hydroelectric/Pumped Storage - In both North America and Northern Europe, systems have been proposed or worked out where wind production can become firm ("firm" meaning that a utility can rely on it) when coordinated with hydroelectric production. With this coordination, hydroelectric plants reduce flow when wind is producing then increase flow when wind dies down; thus water builds up in the hydro reservoir while wind is available, saving it for when it is needed. Denmark currently has such an agreement with neighboring countries and there are proposals for a similar system in British Columbia for the Western US. Such a system requires spare capacity in hydroelectric reservoirs. Alternatively, wind can pump water into a pumped storage reservoir that then outputs power when requested by grid operators. (KEMA, a Netherlands-based international power consulting company has proposed an Energy Island concept that integrates wind with pumped storage in an offshore environment, thereby avoiding variations in rainfall.)
- Currently economic energy production technologies.
- Already used successfully in Denmark/ proposals for British Columbia.
- Could supply baseload, load-following or peak power.
- Requires collocation or coordination of two geographically limited resources: wind and hydroelectric to connected load centers.
- Requires large excess capacity of hydroelectric reservoirs.
- Requires regional or continental coordination between business entities.
- Vulnerable to a coincidence of wind and rain droughts.
3) Solar Thermal Electric (CSP) plus Storage with Wind - Just as wind can help extend the use of hydroelectric resources, wind can also help extend the use of stored heat in a solar thermal electric plant. The dispatchability of solar thermal electric coordinated with a wind resource in the same grid region allows for a smoothed power output of wind and an extension of heat reserves during the night and cloudy weather. To be able to create an ultra-high capacity factor baseload power alternative, the solar thermal plant should probably have in excess of 36 hours heat storage to produce continuously through two cloudy days with available wind. The fact that wind tends to blow during the night respresents an additional complementarity between solar thermal and wind. A similar ensemble between solar thermal and hydroelectric or a triangulation between solar thermal, hydroelectric/pumped storage and wind are all possible.
Offset between timing of wind and sun.
- Both mature or rapidly maturing technologies.
- Can cover baseload, follow load or peak.
- Wind brings down cost of and extends thermal storage.
- Both resources with sufficient transmission can fulfill world energy demand.
- Bringing down cost of STE/CSP and mass thermal storage.
- Collocation of wind and solar thermal in same grid region.
- Coordination between wind and solar generators.
Sending Coal to the Sidelines
There are now viable renewable alternatives to coal and more will be available soon with technological improvements and cost reductions in any one of these technologies. While technological improvements can have a catalytic effect, popular and political will in favor of renewable solutions will help speed getting current and near-term technologies online in the next several years. A commitment to sustainability within the power industry, which has been as addicted as we all have been to fossil and unsustainable fuels, will be an additional factor that will speed the transition to renewable fuels. The coal replacement renewable alternatives require reliance on both the existing knowledge base of the power industry as well as some out-of-the-box thinking.
One can imagine a number of scenarios that can start today, where renewable power will replace the most toxic power plants within a period of years. Regions that have a natural advantage in this endeavor are those near deserts or with substantial hydroelectric capacity. As of today, it is possible to build or plan the development of these resources. Connection to wind resources will enable a more rapid transition to renewable baseload or a majority renewable generation portfolio. Iceland with its exceptional geothermal and hydroelectric is already close to creating an all-renewable energy system.
The short-sighted practice of using valuable and exhaustible fossil fuels to generate electricity on a daily basis will give way to a more suitable use of these power plants, as back-up power. Coal and natural gas will continue to have value in their place on the sidelines as daily electricity demand is met largely by renewable generators. With such a system, we will maintain the reliability of electricity delivery while taking steps towards a cleaner, healthier, more sustainable future.