By Charles Barnhart and Mik Dale
Changing a society’s principle energy resource is a glacial, multi-generational undertaking. The last few transitions occurred naturally. Improvement in availability, energy density and ease of storage ‘fueled’ the transition from wood to coal. Further improvements in energy density, transportability and scale of engine motivated the transition to oil, especially as a transportation fuel. Natural gas represented even greater energy density ease of distribution for municipal heating and lighting and an improvement in conversion efficiency for electricity generation. However, the global climate change wrought by society’s use of oil and coal urge us to transition to a low-carbon energy system before long. This is a different and difficult sort of transition: not eased by energetic advantage and utility but by the desire to increase energy security and mitigate environmental harm.
Coal and oil are amazing energy resources. Storing them simply means piling them up or building a tank. Their energy density–the energy to mass and energy to volume ratio–borders on magical when one considers that the volume of gas contained in a can of soda could transport a car with you and three friends 5 miles down a road. The widespread coal and gas infrastructure permits on-demand conversion of fossil fuels to electricity. Achieving this utility with low-carbon energy systems will challenge our technical creativity and change the way we acquire and use energy.
One such challenge with low-carbon electricity generated from solar photovoltaics (PV) and wind turbines is their dependence on weather. Whereas a coal plant steams along with a pile of coal nearby, a wind turbine’s generation is intermittent and variable. To accommodate wind and PV resources, the power grid will need to become more flexible. A number of technologies could add flexibility to the power grid. Excess generation and increased transmission would insure some generation source could match some demand sink. Today’s fast-acting natural gas “peaker” plants can ramp-up generation during times of peak demand. Smart grid technologies can remotely control energy-hungry appliances and tailor demand to match available supply, rather than supply needing to be adjusted to meet demand. Electrical energy storage stores energy during times of excess generation and delivers energy during times of high demand.
All of these grid-flexibility options require financial capital, material resources, and energy for construction and deployment. The work I do at Stanford’s Global Climate and Energy Project considers all of these requirements, but my fellow post-doc, Mik Dale, and I are particularly interested in the energetic expenditures. We think of energy as a currency. Just as it takes money to make money, we must consume useful energy to acquire useful energy (To note: the 1st law of thermodynamics dictates that one cannot make energy). In an Aristotelian sense, a good energy system is one that delivers far more useful energy than is consumed in its construction and operation. It does not matter how inexpensive, safe or secure it is, if it does not deliver net energy. The question that motivates our research is, “how does incorporating grid-flexibility technologies affect the energy return on energy investment (EROI) for low-carbon energy systems?”
I recently completed an analysis of the material and energetic costs of electrical energy storage. I chose to focus on storage first among grid-flexibility technologies. National and state policy concerning the incorporation of storage on the power grid is rapidly advancing and I recognized a need for a concise comparison of the energetic performance of storage technologies. The comparative metric I developed adopts life cycle assessment techniques and is called the “Energy Stored on Energy Investment” (ESOI). It is a ratio of all the energy stored over the entire life of storage technology to the amount of energy required to acquire the device’s raw materials and assemble them. The greater the ESOI value the less of an energetic drag a storage technology will be on the low-carbon energy system.
The results show that ESOI values for battery technologies are between 2 and 10. ESOI values are greater than 200 for geologic storage technologies like pumped-hydroelectric storage (PHS) and compressed air energy storage (CAES). There are two reasons why geologic storage technologies perform over an order magnitude better than battery technologies. Firstly, they are made from abundant and relatively energetically low-cost materials: earth, water, cement and steel. Secondly, they are very durable and can be charged and discharged tens of thousands of times. This high cycle-life means that the amount of energy stored over its life is much greater than a lead acid battery, for example, which can only be cycled a few hundred times. These results do not mean that we should abandon batteries for grid storage. Rather, we should focus on improving their cycle life in order to improve their ESOI value. Preliminary, unpublished calculations indicate that batteries with an ESOI value greater than 80 or a cycle value greater than about 15,000 would provide a net energy benefit for wind generation over curtailment.
Calculating the energetic costs of energy storage adds to our understanding of low-carbon energy systems in a small but practical way. My group at GCEP strives to calculate the energetic costs of other grid-flexibility technologies and to identify technology pathways to efficient, low-carbon energy systems. Promoting the technological development of low-carbon energy systems by selecting not only for safety, security, and economic viability, but also for high energy return on energy investment values, will provide greater access to energy resources while reducing greenhouse gas emissions.