THERMAL ENERGY STORAGE AS AN ENABLING TECHNOLOGY FOR RENEWABLE ENERGY

THERMAL ENERGY STORAGE AS AN ENABLING TECHNOLOGY FOR RENEWABLE ENERGY Paul Denholm National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO, USA e-mail: paul.denholm@nrel.gov Sean Ong National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO, USA e-mail: sean.ong@nrel.gov ABSTRACT: A variety of electrical energy storage technologies have been proposed to help integrate variable and uncertain renewable energy resources such as solar PV and wind. An emerging electricity storage technology is cold storage for space cooling. Since peak electricity demand in most of the United States is driven by air conditioning, cold storage provides an opportunity to store low value renewable generation occurring during off-peak periods. This includes some unusable mid-day solar generation that can occur at high penetration. Advantages of cold storage include potentially low cost relative to several other storage technologies, as well as very high effective round trip efficiency. A significant disadvantage is that cold storage is tied to a single application (space conditioning) which limits its flexibility. This paper explores the temporal characteristics of space cooling demand and the benefits of cold storage to reduce renewable energy curtailment in the southwestern United States. 1. INTRODUCTION Decreasing costs of renewable electricity have led to increasing use of solar and wind generation in the United States and worldwide. Given the variability and uncertainty of these resources, an increasing concern is the effective use of these resources in the grid. Grid integration studies in the U.S. have found that penetration of up to 35% variable generation (VG) appears technically feasible with modified grid operating practices [1]; however at some point it becomes increasingly difficult to accommodate renewable generation due to a number of factors. One is the limited coincidence between the wind and solar resource and normal demand patterns [2]. This can lead to variable generation being curtailed due to lack of demand during some hours, while meeting only a small fraction of demand during others. A second challenge is the limited flexibility of conventional generators to reduce output and accommodate variable generation resources. At high penetration of solar and wind generation, increased grid flexibility will be needed to fully utilize the variable and uncertain output from renewable generation and shift energy production to periods of high demand [3]. Energy storage is commonly seen as an important enabling technology for large-scale deployment of variable renewable energy sources such as wind and solar. Storage technologies can provide firm system capacity, shift supply of renewable resources to periods of high net demand and provide rapid ramping to address resource variability and the need for additional ancillary services. Challenges of storage deployment include high capital cost and round trip efficiencies that result in the effective curtailment of 2-3% of energy placed into storage [3]. Many energy storage technology assessments ignore the potential use of thermal energy storage (TES) as a method of increasing the penetration of renewable resources. One form of TES commercially available is thermal energy storage for space cooling. Space cooling is a significant use of energy in the U.S., representing about 1% of total demand [4]. Cooling also drives peak electricity demand and need for peaking generation capacity. Potentially a large fraction of this cooling electricity demand could be shifted via cold storage in ice or chilled water. TES has several advantages over conventional electricity storage technologies. Storing thermal energy is often more efficient than storing electrical energy. End use

Generation (GW) thermal storage can achieve very high efficiencies (over 9%) and has the ability to be sited at the load, avoiding losses in the transmission and distribution network [5]. The primary disadvantage of TES is that it is tied to a single application. Cold storage is economically restricted to locations with significant cooling demand. This study examines the system flexibility that can be realized by deploying cold TES focusing on the southwestern United States. 2. CHALLENGES OF RENEWABLE INTEGRATION AT HIGH PENETRATION Figure 1 illustrates several of the challenges of deploying large amounts of wind and solar without enabling technologies such as load shifting or storage. The figure is a simulated dispatch stack in the western U.S., considering a future scenario where PV provides 15% of the annual energy demand and wind provides an additional 15% [6]. The period is May 1-4 based on weather conditions in 25. During each day the large amount of mid-day PV generation forces conventional units to cycle over a large range. Even nuclear plants are forced to reduce output to accommodate PV production. output. Because of these factors, at high penetration of solar, increasing amounts of solar may need to be curtailed when its supply exceeds demand, after subtracting the amount of generation met by plants unable to economically reduce output due to ramp rate or range constraints or while providing operating reserves. The resulting curtailed energy can substantially increase the levelized cost of energy (LCOE) from variable generators, because their capital costs must be recovered over fewer units of energy actually sold to the grid. Figure 2 illustrates the potential impact for renewable curtailment. It uses the REFlex model, which is a reduced form dispatch model designed to examine the general relationship between grid flexibility, variable solar and wind generation, and curtailment [7]. REFlex compares hourly load and renewable resources and calculates the amount of curtailment based on the system s flexibility, defined as the ability for generators to decrease output and accommodate variable generator sources such as solar and wind. It shows simulated dispatch over a 4-day period (April 7-1) in the southwestern U.S. including California, Arizona, New Mexico and Southern Nevada [6]. It demonstrates a case where 1% of the annual demand is met by wind and 2% is met by solar. The figure shows both the simulated solar profile and its contribution to meeting load. Because of relatively low load during this period, PV generation exceeds what can be accommodated using the assumed grid flexibility limits. This typically occurs in the late morning, before the demand increases to its maximum in the afternoon. In these four days about 16% of all PV generation is curtailed and on an overall annual basis about 5% of the PV generation is curtailed. 6 5 Fig. 1: Simulated dispatch of the western U.S. grid where renewables provides 3% of annual demand [6] 4 3 2 Curtailed PV Usable PV Wind Conventionals Load Several factors limit the ability of conventional generators to reduce output to accommodate renewable generation. These include the rate at which generators can change output, particularly in the evening when generators must increase output rapidly in a high PV scenario. Another limitation is the overall ramp range, or generator turndown ratio. This represents the ability of power plants to reduce output, which is typically limited on large coal and nuclear units. Most large thermal power plants cannot be turned off for short periods of time (a few hours or less), and brief shutdowns could be required to accommodate all energy generated during the period of peak solar 1 6 12 18 24 3 36 42 48 54 6 66 72 78 84 9 96 Hour Fig. 2: Simulated system dispatch on April 7-1 with 2% contribution from PV generation and resulting curtailment due to grid flexibility constraints [6] The ability of the aggregated set of generators to rapidly change output at a high rate and over a large range can be described as a grid s overall flexibility. The actual flexibility of the U.S. power system is not well defined. Figure 3 illustrates renewable curtailment rates for one PV 2

PV Curatilment Rate (% of Incremental or Total PV Curtailed) flexibility scenario assuming that the system is able to accommodate PV over a cycling range of 8% of the annual demand range. This corresponds to a flexibility factor of 8%, meaning the aggregated generator fleet can reduce output to 2% of the annual peak demand [7]. This value is based on the Western Wind and Solar Integration Study (WWSIS) study [1] and corresponds roughly to the point where all on-line thermal units have reduced output to their minimum generation levels and nuclear units would require cycling. This value is not intended to be definitive, but is used to represent the challenges of solar and wind integration and the possible flexibility benefits of TES. Figure 3 illustrates the average and marginal PV curtailment rates as a function of PV energy penetration [6]. It should be noted that the x-axis shows penetration of only solar PV. Because wind provides 1%, the total penetration of variable generation is 1% plus the penetration of solar. The average curve shows the total curtailment of all PV at a certain generation level. At the overall assumed system flexibility level, by the time PV is providing 22% of total demand, about 6% of all potential PV generation is curtailed. The actual allocation of curtailment strongly influences the economics of PV and other variable generation. Figure 3 also shows the marginal curtailment rate, or the curtailment rate of the incremental unit of PV installed to meet a given level of PV penetration. If curtailment were assigned on an incremental basis at the point where PV is providing 22% of total demand, only about 5% of this additional PV would be usable, with the rest curtailed. 6% 5% 4% 3% 2% 1% % Marginal Average % 5% 1% 15% 2% 25% 3% 35% Fraction of Energy From PV Fig. 3: Curtailment rates of PV in a base scenario assuming an 8% system flexibility [6] approach of enhancing the use of variable generation. This analysis considers the use cold storage in buildings. 3. GENERATING COOLING PROFILES TES has several advantages over electricity storage devices such as pumped hydro or batteries. Most importantly, it effectively stores energy at higher roundtrip efficiencies. Most electricity storage devices typically store energy with a total round-trip efficiency of less than 8% [3]. Thermal storage efficiency can be closer to 1% [5]. It can also be deployed at the point of use, decreasing need for transmission and lowering transmission losses. The primary disadvantage of thermal storage is that it is tied to an end use, and the demand for cooling varies over multiple time scales. Knowledge of the amount of energy that is shiftable over various time scales is a prerequisite to evaluating the potential large-scale benefits of cold storage. Understanding this hourly and daily variation in demand is particularly important to assess the potential role of TES as an enabling technology for grid integration of VG. This is very different than conventional electricity storage technologies such as pumped hydro or batteries, which are completely controllable and largely independent of weather and human behavior. While modeled and actual data for individual buildings exist, there is less data on the aggregated system demand for cooling and the total system-level potential for TES. In this paper, we use a simple top-down methodology to isolate hourly cooling demand using historical utility loads. By comparing demand on representative mild (low cooling demand) days to other days during the cooling season, we extract an estimate of the hourly cooling demand. We estimate the hourly and total cooling demand across various regions in the United States and compare estimates using our methodology to previous estimates of total cooling demand. Figure 4 demonstrates this difference and provides the basis for our methodology. The figure superimposes the hourly load reported by the Los Angeles Department of Water and Power (LADWP) from May 4, 25, and June 17, 25. If these two days are equivalent in all other factors, the only difference in demand between them should be space cooling driven by temperature. As a result, subtracting the difference is the incremental hourly and total cooling load. A variety of options are available to increase grid flexibility, and a comprehensive analysis of each flexibility option is needed to evaluate the cost-optimal 3

Demand (MW) Cooling Demand (MW) Load (MW) 6 5 4 Estimated Cooling Load Low Demand Day be aggregated to the state or regional level. Figure 6 provides an example of an hourly cooling profile for an entire year for both single utility service territory (Arizona Public Service) and all utilities in Arizona aggregated to the state level. 3 2 12 1 8 Arizona Arizona Public Service 1 6 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 Hour Figure 4. Demand difference within the cooling season (data from LADPW) This approach was repeated by subtracting the total hourly demand from a base or reference day in which there is little or no cooling demand from all other days within the cooling season. Reference (non-cooling) days were generated for weekdays and weekends with a detailed methodology described in [8]. The cooling season for each utility was based on historical temperature and cooling degree data. Temperature data were obtained and assigned to each utility, and the cooling season in the base case is defined as any day with an average temperature of 65 F or higher. This is a conservative approximation that captures the majority of the cooling season. Figure 5 illustrates the base (non-cooling) demand, and cooling demand for NV Energy (serving Las Vegas and Southern Nevada) from June 1 to June 22 25. 6 5 4 3 2 Cooling Demand Base (Non-Cooling) Demand 4 2 1 2 3 4 5 6 7 8 9 1 11 12 Month of Year Figure 6. Example of estimated cooling load patterns for Arizona Public Service and the state of Arizona in 25 4. INCREASING RENEWABLE DEPLOYMENT USING COLD TES Thermal storage extends the contribution of renewable electricity generation by shifting generation to improve its coincidence with normal demand, and by improving system flexibility. System flexibility can be improved by the ability of TES to be a dispatchable load, either adding or reducing demand and providing ancillary services such as operating reserves. This means that fewer conventional generators will be needed to operate at part load during periods of high solar output. The ability of TES to provide load shifting is dependent on the timing of cooling demand and penetration of TES technology. Figure 7 shows the fraction of total hourly demand that is potentially shiftable via cold storage in California and Arizona. The data sets are derived by aggregating the data from individual utilities within each state, and dividing the hourly cooling demand by the total demand in each hour. 1 Figure 5. Base and estimated cooling demand for NV Energy (data from NV Energy) This method was applied to utilities within the Southwestern U.S. for the years 25 and 26. Data can Date 4

Fraction of RE Generation Curtailed Fraction of Demand Shiftable Via Cold TES 6% 5% 4% Arizona California periods of lower cooling demand. Gaining a more thorough understanding of how TES storage can enable greater solar and wind penetration will require detailed production simulations using economic dispatch models currently used by utilities and system operators. 3% 2% 1% % Figure 7. Fraction of demand that could be shifted via cold storage These cooling demand patterns can be compared to periods when wind and solar energy are potentially curtailed, based on simulations described in section 2. Figure 8 illustrates the hourly curtailment rate of wind and solar generation in a scenario where 3% of the electricity in the southwestern U.S. is provided by wind and solar (with a roughly 2:1 ratio of solar to wind) and about 5% of generation is curtailed [6]. 6% 5% 4% 3% J F M A M J J A S O N D Month 5. CONCLUSIONS While it will be some time until solar technologies achieve very high penetrations in the U.S. grid, international experience in wind deployment demonstrates the importance of increasing overall grid flexibility. Key factors in improving grid flexibility include increasing the ramp range and rate of all generation sources and the ability to better match the supply of renewable resources with demand via increased spatial diversity, shiftable load, or energy storage. The use of cold thermal energy storage in buildings provides one option for increased grid flexibility in two primary ways. First, TES allows shifting of the solar resource to periods of reduced solar output with relatively high efficiency. Second, the ability to TES to provide operating reserves can reduce the need for partially loaded thermal generators. Preliminary analysis indicates that a large fraction of curtailment is likely to occur during periods of relatively low cooling demand. This may limit the ability of TES to provide a source of dispatchable load. Additional analysis of the use of TES to provide grid flexibility via reserves and other services will be needed to understand its value in integrating variable renewable resources. 2% 1% % J F M A M J J A S O N D Month Figure 8: Hourly curtailment rate of wind and solar generation where VG provides 3% of the annual demand in the western U.S. [6] In this case a large fraction of curtailment occurs before the major period of cooling demand. However there is some overlap of curtailed generation and cooling demand during May and June. As a result, the benefit of TES to increase renewable penetration may be its ability to provide load shifting during the late spring and early fall, as well as its ability to provide ancillary services during the entire cooling season adding additional grid flexibility. Other enabling technologies will likely be needed to address the supply demand mismatch during 6. REFERENCES (1) GE Energy. (21). Western Wind and Solar Integration Study. SR-55-47434. Golden, CO: National Renewable Energy Laboratory. (2) Denholm, P.; Margolis, R.M. (27). Evaluating the Limits of Solar Photovoltaics (PV) in Traditional Electric Power Systems. Energy Policy (35); pp. 2852 2861. (3) Denholm, P.; Margolis, R.M. (27). Evaluating the Limits of Solar Photovoltaics (PV) in Electric Power Systems Utilizing Energy Storage and Other Enabling Technologies. Energy Policy (35); pp. 4424 4433. (4) U.S Department of Energy. (211) Annual Energy Outlook 211 with Projections to 235 Report #:DOE/EIA-383(211) (5) Willis, R.; Parsonnet, B. (21). Energy Efficient TES Designs for Commercial DX Systems. ASHRAE Transactions. OR-1-16. 5

(6) Denholm, P. and M. Mehos. (211) Enabling Greater Penetration of Solar Power via the Use of Thermal Energy Storage NREL Report No. TP-6A2-52978. (7) Denholm, P.; Hand, M. (211). Grid Flexibility and Storage Required to Achieve Very High Penetration of Variable Renewable Electricity. Energy Policy (39); pp. 1817 183. (8) Denholm, P., S. Ong, and C. Booten. (212) Using Utility Load Data to Estimate Demand for Space Cooling and Potential for Shiftable Loads NREL/TP-6A2-5459. 6