The Impacts of Storing Solar Energy in the Home to Reduce Reliance on the Utility

ARTICLES PUBLISHED: XX MONTH XXXX | VOLUME: 2 | ARTICLE NUMBER: 17001 The impacts of storing solar energy in the home to reduce reliance on the utility Robert L. Fares* and Michael E. Webber There has been growing interest in using energy storage to capture solar energy for later use in the home to reduce reliance on the traditional utility. However, few studies have critically assessed the trade-os associated with storing solar energy rather than sending it to the utility grid, as is typically done today. Here we show that a typical battery system could reduce peak power demand by 8–32% and reduce peak power injections by 5–42%, depending on how it operates. However, storage ineciencies increase annual energy consumption by 324–591 kWh per household on average. Furthermore, storage operation indirectly increases emissions by 153–303 kg CO2, 0.03–0.20 kg SO2 and 0.04–0.26 kg NOx per Texas household annually. Thus, home energy storage would not automatically reduce emissions or energy consumption unless it directly enables renewable energy. 1 n recent years, there has been growing interest in storing energy lead-acid batteries used with solar panels in the UK would increase 41 25 2 produced from rooftop photovoltaic panels in a home battery both primary energy consumption and carbon dioxide emissions . 42 1 3 Isystem to minimize reliance on the electric utility . A number In this paper we critically assess the trade-offs of using lithium- 43 4 of vendors have sought to capture this emerging market, including ion battery storage to capture solar energy and minimize reliance on 44 5 electric vehicle market leader Tesla and German home energy the utility. We build on previous work by using measured electricity 45 2,3 6 storage provider Sonnenbatterie . Notably, Tesla has partnered use and production data from 99 Texas households to understand 46 7 with Green Mountain Power, one of the largest electric providers how adding energy storage would impact power demand, energy 47 8 in the state of Vermont, to offer home storage to its customers; consumption, electricity service costs, and emissions of CO2, SO2 48 9 and Sonnenbatterie has partnered with Sungevity, the largest private and NOx from the electricity system. We consider two different stor- 49 4,5 10 solar company in the United States . age operation models and compare their impacts. We also perform a 50 11 While there is a growing market for home energy storage for sensitivity analysis considering various storage efficiencies, storage 51 12 rooftop solar panels, storage is not strictly required to integrate energy capacities, and storage power capacities to understand the 52 13 rooftop photovoltaic systems with the grid. A study on the impacts impact of energy storage under different scenarios. 53 14 of rooftop photovoltaic panels in California found that even at 100% 15 penetration (measured as the ratio between nameplate capacity and Energy storage system model 54 16 peak system demand), the utility Pacific Gas and Electric (PG&E) The energy storage application considered in this paper is minimiz- 55 17 could maintain adequate voltage levels in its system by increasing ing the interaction between a household and the utility by minimiz- 56 18 the number of transformer tap changing operations at a cost of ing power draws from and injections to the utility grid for the benefit 57 19 US$442,000 annually—or 0.007% of its US$6 billion annual opera- of the electricity customer in terms of increased solar energy self- 58 6,7 20 tion and maintenance budget . These findings align with previous consumption, independence from the utility, and reduced sensitivity 59 21 findings on the impact of high photovoltaic penetration in distribu- to grid outages. This application has been studied extensively in the 60 8 22 tion circuits in California . Furthermore, a number of studies have literature, and is the primary value proposition offered to residential 61 2,3,15–20,22 23 shown that upgrading conductors, upgrading the transformer, or customers by home energy storage vendors . 62 24 incorporating `smart' photovoltaic inverter control could be used We utilize electricity data directly measured from 99 Texas 63 9–14 25 in lieu of storage to maintain adequate system voltage . Even if households over calendar year 2014 to reveal how home storage 64 26 energy storage were needed to integrate rooftop solar panels, it is would operate with solar panels to minimize reliance on the 65 27 not clear that it would have to be installed at the household level. utility. These data track electricity use and solar production with 66 28 Despite the fact that energy storage is rarely required to integrate a one-minute time resolution, allowing us to reveal how storage 67 29 rooftop solar panels, there is significant interest in capturing on- could respond to short-duration power fluctuations. The data were 68 30 site solar generation to minimize reliance on the electricity utility collected on a voluntary basis by the non-profit entity Pecan Street 69 31 and injections of solar energy to the grid. This application has and are freely available to university researchers through an online 70 15–21 26 32 been studied extensively in the literature , and it is the primary portal . Summary statistics for the 99 households in the sample are 71 33 value proposition offered to residential customers by home energy provided in Supplementary Table 1. 72 2,3,22 34 storage vendors . We model home energy storage operation using two different 73 35 While a number of studies have assessed the benefits of energy methods: a `target zero' approach where the battery does not have 74 36 storage that captures rooftop solar energy to mitigate overvoltage information about the future level of solar generation or electricity 75 20,23,24 37 in the distribution grid and hedge utility tariffs , the amount demand and seeks to reduce injections to and demand from the 76 38 of energy consumed by the battery during operation and the grid to zero at all times; and a `minimize power' approach where 77 39 corresponding emissions footprint is typically neglected. One the battery system has perfect information about the future level of 78 Q.1 40 notable exception is McKenna et al.'s 2013 study, which found electricity demand and solar generation over the day, and plans its 79 Department of Mechanical Engineering, The University of Texas, Austin, Texas 78712, USA. *e-mail: [email protected] NATURE ENERGY | www.nature.com/natureenergy 1 ARTICLES NATURE ENERGY ab'Target zero' operation 'Minimize power' operation Calculate target battery power Pbatt(t) = Puse(t) − Pgen(t) Perfect foresight of to zero out net demand Puse(t), Pgen(t) over 24 h Battery energy Yesconstraints No Optimization violated Minimize sum of squares of Puse(t) − Pgen(t) − Pbatt(t) over 24 h Pbatt(t) = 0 Pbatt(t) = Puse(t) − Pgen(t) Subject to battery energy and power Battery power constraints constraints violated Yes No Pbatt(t) = Prated Pbatt(t) = Puse(t) − Pgen(t) Pbatt(t) over 24 h 6 6 Use Battery Use Battery Solar Net Solar Net 4 4 2 2 0 0 Power (kW) Power (kW) −2 −2 −4 −4 27 July 27 July 27 July 27 July 27 July 27 July 27 July 27 July 27 July 27 July 0:00 5:00 10:00 15:00 20:00 0:00 5:00 10:00 15:00 20:00 Figure 1 | Storage operation model control logic and sample outputs. a, The ‘target zero’ method steps from one minute to the next with no foresight of future electricity demand or solar generation, and seeks to set net grid demand to zero whenever possible without violating the battery’s energy and power constraints. b, The ‘minimize power’ method has perfect day-ahead foresight of electricity demand and solar generation, and uses an optimization program to minimize the sum of squares of net grid demand over the entire day. Sample outputs for each method are shown below the flowcharts, with storage discharging given a negative sign and storage charging given a positive sign. 1 operation to minimize the sum of the squares of net power demand utility as much as possible. Neither operational method explicitly 26 2 from the utility over the entire day. Besides the level of foresight, the considers other grid-level services that could be offered by 27 3 primary distinction between these two methods is that `target zero' distributed energy storage or the potential economic benefits of 28 4 seeks to maximize the number of hours during which the household those services. While we are aware of the fact that the methods of 29 5 is completely independent from the grid, while `minimize power' storage operation selected are not optimal from a purely economic 30 6 seeks to minimize the magnitude of grid power demand over every or system perspective, our objective is to assess the specific impacts 31 7 minute of the day with equal weights placed on each minute, so that of storing solar energy in the home to minimize reliance on the 32 8 the household is resilient to a grid outage regardless of when it oc- utility, because this application is the primary value proposition 33 2,3,22 9 curs. Furthermore, the `target zero' mode restricts the battery system offered to residential customers by storage vendors . The details 34 10 to charge only with solar power, while the `minimize power' mode of each of these operational models are provided in the Methods. 35 11 allows the battery to charge with grid or solar electricity to minimize For both operational models, three parameters define the home 36 12 demand over the day.

Load more