Much has been written about renewable energy, but few stories have focused on the complexity of determining the optimal mix of solar and wind generation, and the kind and amount of energy storage, that implementation of renewable portfolio standards will require. This article describes a protocol to do that. The findings show that the amount of required storage will be several times greater than most people in the power industry believe, and that prevailing weather conditions are a critical component of the decision-making.
Climate change concerns have resulted in state governments issuing regulations that require increased use of renewable energy such as solar and wind power. At least 29 states and the District of Columbia have passed renewable portfolio standards (RPSs), and eight additional states have set non-binding renewable energy goals. These regulations vary considerably, with some RPS requirements applying only to investor-owned utilities, and some including municipalities and electric cooperatives. Fifteen states have requirements for 25% or greater renewable energy within the next 10 years, and seven states have requirements for 50% or greater with timeframes longer than 10 years. This situation, together with uncertainties surrounding lithium-ion (Li-ion) batteries and the availability of suitable alternative storage technologies makes investment decisions difficult for all stakeholders.
Currently, limited renewable energy is incorporated into the grid through the use of conventional energy operating strategies, which account for the generation limitations of solar and wind. However, as the requirements for renewable energy increase, adequate and reliable storage will become a critical necessity because solar and wind power are intermittent. A methodology to determine the necessary type and size of renewable energy generation and storage requirements to meet customer expectations of reliable electricity 24 hours a day under all weather conditions is needed. The insights that follow are the result of such a methodology.
A Renewable Energy Generation and Sizing Methodology
Solar power is only generated during daylight hours and is significantly affected each day and over the year by cloudy and partially cloudy days. Wind speed can also change drastically over the course of the day and with long-range weather patterns. Both solar and wind variability significantly affect the sizing of a specific renewable energy power facility. The charging and discharging efficiency and parasitic power loss associated with the storage component not only affect the size of the storage facility, but also that of the solar or wind farm.
The existence of these variabilities, which do not exist with conventional power sources, suggests that the analysis of renewable energy storage must be done over short periods to assure that renewable energy and associated storage are adequate to meet the grid load at all times, while meeting the regulatory requirements under all weather conditions. In this article, a proprietary model utilizes a methodology that considers factors such as grid load, RPS requirements, type of storage facility, charge/discharge efficiency, solar generation profile, typical cloud conditions, and wind speed profile, among other things. The insights that follow are based on typical equipment operating characteristics, weather conditions, and capital investment for the solar farms, wind farms, and various storage technologies.
The analysis for a specific site is unique because it must take into account the expected prevailing weather conditions at the location over the course of a year. Some of the characteristics that must be considered in making renewable energy investment decisions include the size and type of the installation, expected weather conditions, capital costs, operating costs, equipment reliability, and maturity of the technology. The development and demonstration status of storage technologies is a critical consideration in utility industry short- and long-term planning.
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