by Davis Swan
There is a consensus in many countries that burning coal to generate electricity is something that needs to be phased out as quickly as possible. The Clean Power Plan in the U.S. has that as one of its most likely outcomes and there have been explicit commitments to retire coal-fired generation plants by governments all over the world.
When considering the options for replacing the electricity generated by coal-fired plants there are two characteristics of these plants that need to be considered. The first is that coal is the cheapest and most abundant non-renewable fuel available. The second is that coal-fired plants are very reliable – more reliable even than natural gas-fired plants because they can stockpile fuel on site so that they are not subject to pipeline congestion problems. And getting approval to build new pipelines is not easy these days.
One of the strategies for replacement of coal-fired generation is the development of more wind and solar power. This approach is not without its problems because of the inability to store energy from these sources which are often not available during peak demand times of the day. Matching the 24×365 reliability of coal-fired plants using renewables would be very challenging.
When you think about it the only thing wrong with coal-fired plants is the fact they burn coal to produce the steam used to drive turbines. If a renewable source of heat could be supplied to these plants they could continue providing reliable power and the negative aspects of burning coal would be eliminated.
In jurisdictions where renewable energy sources have been developed extensively the disconnect between electricity production and system load is starting to become problematic. For example, on many circuits on Oahu the amount of electricity generated by roof-top solar panels actually exceeds system demand mid-day some days. Although there is plenty of potential to expand solar power in Hawaii from a resource standpoint it will not be possible without the ability to time-shift production to match demand through the use of energy storage. As a result solar panel permits have been falling for the past two years.
In Denmark, where the nameplate capacity of wind turbines is approximately 1/3 of total generation capacity in the country, wind generation frequently exceeds domestic demand which requires the export of the excess to neighbouring countries. Obviously if all of Denmark’s neighbours also developed a similar amount of wind capacity there would be nowhere to export the electricity to. Texas and parts of the American Mid-West are facing similar issues.
So we are faced with two different problems;
- The need to stop burning coal to generate electricity
- The need to store excess electricity generated from wind and solar
Fortunately, there is a combination of field-proven technologies available today that can solve both problems. I will refer to this combination of technologies as “Thermelectric Power”.
Thermelectric Power provides a large rapid response load which can be used to stabilize the grid when there are variations in renewable energy generation. It also stores renewable energy by converting it to thermal energy.
The mechanism for storing the energy is molten salt â€“ a mixture of 60 percent sodium nitrate and 40 percent potassium. Thermal Energy Storage (TES) systems using molten salt have been used for more than 10 years as a way to extend the hours that Concentrated Solar Power (CSP) plants can deliver electricity.
The initial research was done at the Sandia National Solar Thermal Test Facility in New Mexico. The first large-scale commercial application of the technology was at the 50 MW Andasol CSP in Spain which came on-line in March, 2009. The Solana CSP plant commissioned in the fall of 2013 in Arizona includes the largest TES facility deployed to date, able to produce 280 MW of electricity for up to 6 hours after sunset.
Excess wind or solar generated electricity can be used to heat the molten salt to a temperature of more than 1,000 degrees Fahrenheit using industrial electric heating elements. During peak demand periods the molten salt would be circulated through a heat exchanger to transform water into the steam required to power conventional steam turbines. The infrastructure to support the conversion of thermal to electrical energy by means of steam turbines exists at every coal-fired electrical generating station which allows the re-use of these very expensive components with only minimal modifications.
Both the heating of the molten salt and the use of molten salt to generate electricity using steam turbines are proven technologies that are deployed today. By integrating Thermelectric Power into an existing coal-fired generation station it would be possible to phase out the burning of coal as more and more wind or solar generation is developed. This approach would also maintain energy security because it would be possible to switch the power source back to coal for short periods of time to deal with extended periods of calm winds. This dual source approach minimizes both CO2 emissions as well as any risk of power failures on a grid where the primary sources of electricity are renewable.
The cost to implement molten salt storage at an existing coal-fired plant would be $250-$350/kwh. This is a fraction of the cost of utility scale battery storage. More importantly molten salt storage does not suffer degradation in capacity over time. The molten salt can be heated and cooled over and over again so that the service life of this technology is measured in decades.
Thermelectric Power could transform the more than 500 coal-fired generating stations in the U.S. into “green” energy sources. More than 10% of those plants are combined heat and power (CHP) plants on University and College campuses. Students and faculty have been actively protesting to stop the burning of coal at these plants for years.
As rate-payers, tax-payers, and advocates for a sustainable energy future we have a choice to make.
We can demand that coal plants be decommissioned and dismantled at a cost of billions of dollars. That choice would require the construction of natural gas-fired plants or nuclear plants with approximately the same generation capacity in order to handle peak loads in the evening when winds are calm – construction that would require more billions of dollars and would continue to emit vast amounts of CO2 annually.
Or we can demand that our coal plants be converted to Thermelectric Power which would dramatically reduce the amount of coal being burnt to generate electricity. Coal would only be used as a fuel when electricity generation from renewable sources was not available for extended periods of time. But the flip side of that is that coal could be used in that way to back up renewable generation. As a result we could develop as much wind and solar energy as we wanted without worrying about dealing with excess when demand is low and without worrying about destabilizing the grid.
A future fueled by renewable energy is possible using technology that is available today. We just need to want it enough to make it happen.
JC note: As with all guest posts, please keep your comments civil and relevant.