2. Energy Storage Technologies
Among others Chen et al (2009), Ibrahim et al (2008), Hadjipaschalis et al (2009) have thoroughly explored the state of ES technologies in the present and near-future. Their analysis roughly divides ES technologies into two categories (see Table 1), those that are useful for power quality management (capable of making short-term, high power, low energy interventions) and those that are useful for energy management (capable of mediating variations in supply and demand). Though power quality management (a.k.a. “Ancillary”) ES technologies have a definite role to play in the future stability of the electricity grid (e.g. Shayeghi et al 2009 and Hartikainen et al 2007), their likely contributions seem difficult to quantify in an absolute manner. Therefore, this study seeks to quantify the potential effects energy management (a.k.a. “Bulk”) ES could have on GHG emissions in the near-term while discussing qualitatively the advantages and drawbacks of the use of ES for power quality management with regard to GHG emissions.
Table 1: This table categorizes the energy storage technologies reviewed by Chen et al (2009) into either Power Quality Management or Energy Managment energy storage based on their applications.
Power Quality Management ES
|
Energy Management ES
|
Capacitors
|
Pumped-Hydro*
|
Super-capacitors
|
Compressed-Air*
|
Superconducting Magnetic ES
|
Thermal ES
|
Flywheels
|
Batteries (NaS*, ZEBRA, Li-ion)
|
Batteries (Lead-Acid, NiCd )
|
Flow Batteries (VRB*, ZnBr, PSB)
|
Fuel Cells
| |
Solar Fuel
|
*Chen et al (2009) refers to proven MW-scale, multi-hour systems operating
By focusing on the “near-term” (in this case approximately the next two decades) means that some consideration must be given to the fact that some technologies are not yet viable on a commercial (large-) scale (Chen et al 2009). In fact without even considering cost limitations, only pumped-hydro power, compressed-air, and certain types of batteries and flow-batteries have examples of successfully developed MW-scale systems capable of operating for multiple hours. The remainder of this section briefly overviews bulk and ancillary ES technologies as well as the one type of ES-related smart-grid technology (specifically plug-in EVs). For a broad compilation of the physical and performance attributes of ES technologies please see the table in Appendix B.
a. Bulk Energy Storage
Various forms of bulk ES have been employed by electric utility companies since the beginning of the 20th century (Chen et al 2009). Not surprisingly, there are a variety of different types of bulk ES technology at different stages of development in use across the world today. After reviewing numerous academic articles and observing internal industry presentations on the topic, it is apparent that it is technologically feasible to incorporate substantial amounts of bulk ES on developed electric utility grids within the next two decades. As this assumption predicates the usefulness of this study, a review of these technologies is in order. However, rather than attempting to describe all potential ES technologies that may be employed by electric utilities, this brief review prudently highlights the three types of bulk ES that are in the latter stages of technological development and already have examples of multi-MW scale plants in regular operation somewhere in the world. While the three types outlined below (pumped hydro, compressed-air ES, and sodium-sulphur batteries) do not create an all-inclusive list of the possible bulk ES technologies (e.g. certain types of flow batteries and flywheels could have also been included) that could be included in this study, it is believed that those reviewed below should sufficiently validate the incorporation of ES technologies on developed electricity grids over the next two decades.
This review will not focus on underdeveloped ES technologies (e.g. hydrogen-based fuel cell systems), because it is unlikely that they will be widely installed at a substantial capacity prior to the latter quarter of the next two decades. This decision should avoid basing the remainder of the study on speculation about research and development timelines that may or may not materialize. Similarly, this review will not cover mature technologies that are unlikely to be employed at a significant scale within the foreseeable future due to practical limitations (e.g. Pb-acid and NiCd battery systems). Finally, this review will not focus on so-called thermal ES technologies, which heat or cool a medium during times of low-demand in order reap the benefits of the temperature difference during high-demand times (e.g. systems that freeze water overnight to assist in cooling buildings during the day). Admittedly, the use of the technologies in this last category could result in many of the same benefits with regard to reduced GHG emissions as the bulk ES technologies that are being reviewed. However, such technologies do not tend to produce electricity at the end of their charge/discharge cycle. Thus, they will only be employed by end-users of the electric utility grid. Whereas, the technologies being reviewed are able to both consume and produce electricity. This allows them to be used either by end-users or utility companies.
Aside from perhaps small-scale Pb-acid battery systems, the use of pumped hydroelectric storage (or simply ‘pumped hydro’) is perhaps oldest and most well-developed bulk ES technology in use today. Chen et al (2009) site examples of pumped hydro being used by electric utilities as early as 1929 and note that more than 100 GW of pumped hydro capacity is installed across the world today. Ibrahim et al (2008) explain that one of the main advantages of pumped hydro is the technology’s availability. Described simply, when electricity demand is low, electric pumps draw water from a lower reservoir and pump it to an upper reservoir. Then, when electricity demand is high, water flows from the upper to the lower reservoir through hydroelectric turbines similar to conventional hydroelectric dams (Ibrahim et al 2008). The use of reversible pump/generator assemblies acting as both pump and turbine is also possible (Hadjipaschalis et al 2009). Typically, pumped hydro plants use two naturally occurring or artificially constructed bodies of water as the reservoirs; however, Hadjipaschalis et al (2009) suggest that abandoned mines can also provide a suitable venue for the lower reservoir.
Due to the physical nature of pumped hydro (i.e. storage capacity is linearly proportionate to the height difference between the reservoirs and the amount of water stored), Baker (2008) states that the opportunities for significant advances in pumped hydro technology are limited. Recent reviews show rough agreement regarding the overall cycle efficiency of pumped hydro, which is listed as 71-85%, 65-80%, and 70-85% (for Chen et al 2009, Ibrahim et al 2008, and Hadjipaschalis et al 2009 respectively). The primary sources of inefficiency are losses from evaporation at exposed water surfaces and electrical conversion loses (Hadjipaschalis et al 2009). Thus, pumped hydro is considered a mature technology and its cycle efficiency is unlikely to change over the next two decades. Furthermore, an industry expert states that in the US it typically takes 7-10 years to take a pumped-hydro plant from initial conception to full operation (Boston and Mansoor 2010). This means that if efforts to install pumped hydro begin in earnest within the next few years, a substantial amount of new installed-capacity could realistically be operational well before the end of the next two decades.
Stressing the need for more installed ES within the immediate future the same industry expert suggests that compressed-air ES (or CAES) plants can be operational within a 3 year time period in the US (Boston and Mansoor 2010). CAES is typically considered to be a developed technology; however, most studies (e.g. Baker 2008 and Cavallo 2007) acknowledge only two operational multi-MW plants in the world (i.e. a 290 MW plant in Huntorf, Germany, installed in 1949 and a 110 MW plant installed in the 1970s in Alabama in the USA). Since newer, larger CAES plants are currently in the planning stage in the US (e.g. First Energy Generation is planning to build a plant in Norton, Ohio, USA, that could potentially be capable of running at 2700 MW; see Leidich 2010), it is not unreasonable to consider the potential of CAES in this study.
Ibrahim et al (2008) explain that large-scale CAES utilizes underground caverns made out of solid rock to store air at high pressure. These caverns can be “created by excavating comparatively hard and impervious rock formations, salt caverns created by solution- or dry-mining of salt formations, and porous media reservoirs made by water-bearing aquifers or depleted gas or oil fields, e.g. sandstone and fissured lime” (Chen et al 2009). The air is cooled and compressed into the cavern during times of low-demand using electric compressors (Hadjipaschalis et al 2009). During times of high-demand, the air is released from the cavern, heated, and combined and combusted with natural gas to retrieve the stored energy and produce electricity (Ibrahim et al 2008). Ibrahim et al (2008) note that for every 1 kWh produced using CAES approximately 0.7-0.8 kWh energy was used to compress air during low-demand and 1.22 kWh of natural gas is combusted. By way of comparison, Jaramillo et al (2007) suggest that conventional natural gas power plants are 28-58% efficient; this means that in a non-CAES plant 1.72-3.57 kWh of natural gas would need to be combusted to produce 1 kWh of electricity.
Recent articles suggest that the overall cycle efficiency of CAES is approximately 70-80% and 70% (see Chen et al 2009 and Ibrahim et al 2009 respectively). This is comparable (or slightly lower than) the cycle efficiency of pumped hydro; however, Hadjipaschalis et al (2009) point out that the self-discharge rate of CAES systems are minimal meaning that energy can be stored for months or years without significant additional losses. On the other hand, Chen et al (2009) state that CAES can only be used in association with natural gas plants. Thus, committing to CAES as a long-term strategy relies on the availability of natural gas or synthetic substitutes and requires accounting for some additional considerations (see Cavallo 2007). However, with regard to meeting the need for ES over the next two decades, CAES is another appropriate candidate.
While pumped hydro and CAES store energy in mechanical form, sodium-sulphur (NaS) batteries store energy in chemical form. Unlike conventional batteries (e.g. NiCd batteries), which store chemical energy in a solid form, NaS batteries consist of molten sulphur (on the positive electrode) and molten sodium (on the negative electrode) divided by a ‘solid beta alumina electrolyte’ (Hadjipaschalis et al 2009). The electrolyte allows the sodium ions to pass through the electrolyte during the charging and discharging phases as electrons pass through an external circuit (Hadjipaschalis et al 2009).
Baker (2008) explains that NaS batteries have 100% coulombic efficiency meaning that all of the electricity stored in the battery can be recovered, yet there is some disagreement regarding the overall efficiency of the system. Hadjipaschalis et al (2009) suggest that the heat produced during the charging and discharging phases is sufficient to maintain the battery’s operating temperature of 300-350ºC. However, Chen et al (2009) suggest that an additional heat source is needed, which reduces the overall performance of the system. This discrepancy leads to a difference in overall cycle efficiency (i.e. 89-92% or 75-90% according to Hadjipaschalis et al 2009 or Chen et al 2009 respectively). In either case, it appears that NaS batteries have higher cycle efficiencies than most pumped hydro and CAES technologies. Additionally, NaS batteries are expected to have a cycle life of approximately 2,500 cycles and have been proven to maintain a constant, multi-MW discharge of approximately 8 hours. Thus, NaS batteries have also been considered a viable bulk ES option in this study.
b. Ancillary Energy Storage
While bulk ES technologies provide the opportunity for peak shaving and load leveling, which may significantly improve the efficiency of traditional generation plants and allow intermittent renewable generation systems to meet consumer demand, there are other ES technologies that may be better suited to meet the ancillary service needs of the grid, notably frequency regulation as described in section II.1. Traditionally, fossil fuel power plants are used to meet regulation needs; however, new ancillary ES plants are able to respond to regulation much more precisely than fossil fuel power plants (Shelton 2010).
Similar to bulk ES development, ancillary ES technologies are at various stages of development and deployment. Many ancillary ES technologies (e.g. superconducting magnet ES and ultracapacitors) are underdeveloped and are not currently ready for mass deployment (Pickard et al 2009). Other ancillary ES technologies (e.g. some battery systems and high-speed flywheels) already have several multi-MW plants in operation across the world today (e.g. Shelton 2010 and Capp 2010). However, the amount in operation today by no means covers a majority (or even a large minority) of the need. Such technologies are considered developed but under-deployed (or otherwise in the ‘pilot’ stage of deployment). Thus, given the appropriate incentives, ancillary ES could be brought into wider deployment soon (Jackson 2010).
The remainder of this study has been predicated upon the assumption ancillary ES technologies such as batteries and high-speed flywheels, which meet and exceed that ancillary service standards established by US regulatory authorities (McIntosh 2010), could feasibly compensate for any loss in regulation services caused by a reduction in traditional generation capacity. Even though these effects are not explicitly included in the calculations, Østergaard (2006 & 2008) highlights the need for such considerations as intermittent renewable generation such as wind replaces traditional generation. Since the need to ensure grid-stability through ancillary services such as frequency regulation is vitally important to the viability of any ES-related intervention, the following section illustrates an alternative solution to meeting that need through the use of smart-grid technology and an aggregation of smaller-scale ancillary ES.
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