| 08 February 2012
Energy storage is becoming an important aspect of our grid system as renewable energy sources replace more conventional, consistent, carbon-heavy power. Utilities can operate most efficiently from both an energy and financial standpoint, by following the demand curve as closely as possible. When intermittent electricity sources like solar make up more of the power supply mix, utility operators have less control over how much power is produced. This is one of the largest hurdles to increasing renewable grid penetration and it causes utility managers to have to constantly be prepared for dips in renewable output.
Pumped hydro and solar thermal storage can both increase the capacity of solar to support our growing needs.
Running back-up generation, usually using natural gas, means that at least some of the time, more energy is being produced than is needed, which leads to throwing away some of the renewable power generated. This leads to an increase in the cost of electricity. At present, scientists, engineers, policy makers and entrepreneurs are all working to gain more control over renewable sources, to dispatch the electricity immediately when it is needed, and to save the excess for when the sun isn’t shining or the wind isn’t blowing.
To improve the ability of solar power to provide consistent power, day and night, on a large scale, it will need to be coupled with large scale, long-term energy storage that can charge up during the day and discharge at night. Two technologies that address these issues are pumped hydro and solar thermal storage, both of which can increase the capacity of solar to support our growing needs.
Pumped hydro
For large-scale, long term storage, pumped hydro energy storage (PHES) is the oldest, most efficient, and most successful technology. The way it works is that excess energy is used to pump water uphill. When the electricity is needed, the water is allowed to flow back downhill and the pump acts as a turbine, spinning to generate electricity rather than using it. In the United States, there are 42 PHES facilities with a total capacity of about 23GW and worldwide the capacity at 130GW. Recognizing that storage will become more important as more renewable energy enters the grid, there has been a surge in interest in pumped hydro projects. Worldwide, there is an additional 76GW of PHES proposed to be installed by 2014.
Unfortunately, PHES presents many of the same environmental and siting concerns as dam hydropower, such as disturbing aquatic habitats, reducing downstream water flow, and habitat destruction by flooding. In order to alleviate such environmental harm, many of the proposed projects are relying on novel approaches to pump and store water. For example, more than a quarter are using underground caverns, quarries and pit mines as reservoirs to reduce the impact on wildlife. In the United States, of the 36 proposed projects that have been granted preliminary permits, less than a quarter would involve damming new rivers.
While PHES has not yet been directly tied into solar projects, solar developers are beginning to see the benefit of coupling their projects to pumped hydro. Near Palm Desert, California, the proposed Eagle Mountain Pumped Storage project will take advantage of four nearby solar arrays under development whose capacity will total more than 2,500 MW.
Solar thermal storage
Utility-scale solar thermal is another technology emerging to combat the inconsistency of solar energy. At a solar thermal facility concentrating mirrors focus the sun’s rays at a fixed point, generating extremely high temperatures, between 500° to 900° F, used to melt salt stored at the focal point. Because the melting point of salt is so high, it is able to store a great deal of energy in its melted form and release it when cooled. The molten salt can be used to heat water into steam, generating electricity by turning a turbine or it can be insulated and stored until the electricity is required.
Solar thermal storage appears on the verge of dramatic rise, with the first of 15 small solar thermal plants opening in Thailand and in California. BrightSource Energy added molten salt storage to their power purchase agreement with Southern California Edison, creating a cost recovery mechanism that will allow BrightSource to monetize the benefits of providing consistent, dispatchable solar-generated power to the utility.
Spain is leading the way with solar thermal storage with the Andasol Plant in Andalucía, Spain and the Gemasolar facility owned by Torresol Energy Investments, which was able to demonstrate providing electricity for 24 hours last July.
The Gemasolar Facility, located near Seville, Spain, focuses sunlight from 2,650 mirrors to the top of a 140 m tower where it heats molten salt, reaching temperatures of 900 F. Image credit: Torresol Energy
Traditional concentrating solar facilities employed without thermal storage are estimated to have capacity factors of about 20%, meaning the plant produces about 20% of the theoretical maximum power it could produce were it running at 100% capacity, 24 hours a day. Including solar thermal storage that capacity is increased to a staggering 70%. The benefit of concentrating solar power when used in conjunction with thermal energy storage is that it not only increases the efficiency of the facility, but it also acts as an “’enabling’ technology for wind and solar generation,” according to Paul Denholm and Mark Mehos, two scientists at the DOE’s National Renewable Energy Laboratory in the report “Enabling Greater Penetration of Solar Power via the Use of CSP with Thermal Energy Storage”.
For utilities employing storage, renewables start looking more like traditional forms of base load electricity.
It is estimated that when more than 10% of energy on the grid is coming from solar, the value of the power produced drops dramatically because it cannot be relied on to produce consistent power. As a result, back-up plants, usually natural gas, must be employed for those times when renewables are not producing. Implementing storage, whether of traditional, carbon-based energy or renewable energy, returns the value to renewable capacity by ensuring that energy will be available when needed. For utilities employing storage, renewables start looking more like traditional forms of base load electricity such as nuclear or coal, making them much more attractive not only to the environmentally conscious consumer, but to the utility managers, too.
Written by Sydney Kaufman, Contributing Editor - US, Solar Novus Today
