Integrated Wind, Solar, and Energy Storage

Colocating wind and solar generation with battery energy storage is a concept garnering much attention lately. An integrated wind, solar, and energy storage (IWSES) plant has a far better generation profile than standalone wind or solar plants. It results in better use of the transmission evacuation system, which, in turn, provides a lower overall plant cost compared to standalone wind and solar plants of the same generating capacity. IWSES plants are particularly suitable for regions that have set high targets for wind and solar generation but have limited land available for project development.

The Concept of an IWSES Plant

Separate wind and solar plants connected to the same point of interconnection do not constitute an integrated wind and solar plant. In an IWSES plant, wind turbines, photovoltaic (PV) solar arrays, and a battery energy storage system (BESS) are integrated into a single plant using state-of-the-art controls. It is noteworthy that the integration of wind and PV solar plants, as well a BESS, can be performed at the wind turbine/PV array level or the farm level. In farm-level integration,

Designing Plants with a Better Generation Profile and Lower Overall Cost 

balance-of-plant (BOP) equipment such as transformers and switchgears, as well as the upstream transmission evacuation system, is shared by the wind turbines, solar arrays, and BESS. In turbine-level integration, each converter may be potentially shared by a wind turbine, solar array, and BESS. Benefits of an IWSES Plant An IWSES plant may offer several benefits over standalone wind or solar plants of the same capacity, some of which can be readily monetized while others cannot. If we are to understand such plants’ economic value, then these benefits must be quantified. Here, we first present the benefits of integrating wind and solar generation; we then discuss the benefits accrued from the addition of energy storage.

 Benefits from Integrating Wind and Solar Generation The key benefits of an integrated wind-solar plant relative to standalone wind and solar plants with the same cumulative capacity are as follows: 

  Decrease in project development costs: For wind and solar projects, project development studies are required to determine project feasibility, socioeconomic impact, and impacts on the market, the environment, and the grid itself. Wind and solar projects also necessitate the fulfillment of legal and regulatory requirements, such as securing permits and licenses, land-lease agreements, environmental impact assessments, construction permits, grid interconnection agreements, and power purchase agreements. A reduction in project development costs can be achieved through efficiencies in project studies and filings with the statutory bodies because they are the same for both types of generation. 


 ✔ Better use of available land: Colocating wind and solar power plants conserves space and increases the energy density (i.e., the amount of energy produced per acre of land). In many wind plants, the land between the turbines is often left unused. To make better use of space, wind and solar plants can be colocated, provided that conditions for both wind and solar power generation are favorable at the same location. Alternatively, if wind and solar plants can be sited adjacent to one another, they can share the same transmission evacuation infrastructure.


   Complementary generation profile: A further benefit of integrating wind and solar generation stems from the complementary nature of both the diurnal and seasonal patterns of their generation. •   Diurnal: Figure 1 shows the average hourly generation during selected months for a wind-solar plant made up of roughly 500-MW each of wind and solar generation capacity. As the figure makes clear, in general, wind generation dips during the day and increases late in the evening. On the other hand, solar generation follows the sun and is highest during the middle of the day. Combining wind with solar reduces the difference between the generation levels during daytime and nighttime periods.


•         Seasonal: Figure 1 also shows how wind generation at this site is highest during the monsoon months (June– September); this is also the period during which solar generation is at its lowest due to more incidences of cloudy days.

   Potential savings in transmission evacuation costs: An integrated wind-solar plant also has the potential for savings in evacuation and transmission upgrade costs. Typically, in a wind or solar plant, the collector system is designed to carry the maximum output of the plant. For example, standalone 100-MW wind or solar plants would each have 100 MW of evacuation capability. However, an integrated plant can make do with an evacuation capability lower than 200 MW because wind and solar generation will not peak at the same time. Figure 2 shows a range of cumulative hourly generation during selected months for a 1,000-MW wind-solar plant. Here, the blue line indicates the average cumulative hourly generation. In addition, the red and green lines in Figure 2 indicate the bounds within which the cumulative generation for each hour would occur 95% of the time. It is, thus, possible to design the evacuation system with a rating lower than the sum of the maximum capacities of the wind and solar plants. In the rare event that the cumulative   


generation exceeds the rating of the evacuation system, the output of the wind or solar plant could be curtailed to keep the power flow from exceeding the transmission limit. ­Similarly, integrating wind and solar plants can reduce the need for upgrades to the transmission system.

  Sharing of operations and maintenance (O&M) expenses: Another key benefit of an IWSES plant is the reduction in O&M expenses. Fewer maintenance personnel are required to maintain an integrated wind-solar plant compared with those needed to maintain separately located wind and solar plants. The skill sets needed to maintain wind and solar plants are similar—in particular, the skills required to maintain power conversion equipment and the switchyard. On the operations side, savings could be achieved by sharing the control room, communication infrastructure, and personnel necessary for plant operation. Benefits from Integrating a BESS A BESS connected to the bulk power system can provide several generation- and transmission-level services such as energy arbitrage, reserves regulation, transmission congestion management, and transmission asset deferral. With aggregation and suitable regulatory changes, a customer-sited BESS can perform additional customer-level services as well as upstream services such as distribution-level and bulk power system services. However, a BESS integrated with a wind solar plant can also provide many plant-level services mandated by grid codes or regulations or incentivized through markets. The stacking of plant-level and system-level services can help derive more value from energy storage. A BESS integrated with a wind-solar plant can provide many of the same system-level services as those provided by a standalone BESS, such as the following:

  Energy shifting: Energy storage is frequently used for energy arbitrage or load shifting. In the United States, pumped storage hydro plants were installed mainly to integrate nuclear base-load power plants, charging at night when energy prices are low and generating during the day when prices are high. Battery energy storage has been used to provide energy during peak load hours in several niche scenarios. It will be possible to use battery energy storage to a greater extent as costs decline in the future.

  Ancillary services: Frequency regulation (i.e., the rapid adjustment in generation or load to balance the system in the seconds-to-minutes time frame) is one of the few applications that can provide full cost recovery for a commercial battery storage project. However, ancillary services such as regulation are not explicitly procured and compensated for in all systems. BESSs have recently found a niche in balancing ancillary services such as fastfrequency response. A BESS can also be used to provide black-start capability to the system and support voltage on the bulk power system in addition to balancing ancillary services, as discussed previously.  

IWSES plants are particularly suitable for regions that have set high targets for wind and solar generation but have limited land available for project development.

Transmission and distribution congestion management and upgrade deferral: Depending on its location, a BESS can alleviate transmission congestion by serving the demand in the transmission-constrained area. The BESS can be used to defer the need for transmission or distribution system upgrades by time-shifting energy from low- to peak-demand periods. A BESS integrated with a wind-solar plant can provide additional plant-level services. These plant-level services may help the plant comply with grid codes or operational requirements. While these services can be provided by a BESS that is not integrated with the plant, more control and coordination would likely be required to achieve the same results.

  Compliance with technical standards: The grid codes in various countries require wind and solar plants to limit their ramp rates and, in some cases, also provide primary frequency response. It may be economical for an integrated BESS to provide these services rather than curtail the generation of the wind and solar plants for this purpose.

  Compliance with operational requirements: In addition to facilitating wind and solar plants’ compliance with technical standards, a BESS can also make it easier for a plant to meet operational requirements, for instance, in the firming (reducing forecast deviations) of wind and solar generation. In some countries, wind plants are penalized if generation deviates from the forecast by more than a certain percentage. As it reduces forecast errors, a BESS can be used to lower the penalties imposed on wind plants.  

Designing an IWSES Power Plant

Recently, a techno-economic feasibility study for IWSES plants was completed for two sites in India (Ananthpur, Andhra Pradesh; and Kutch, Gujarat). This study was performed for the Infrastructure Leasing & Financial Services Energy Development Company Limited, a developer and financier of renewable projects in India under a technical assistance grant from the U.S. Trade and Development Agency. The deliverables of this study were as follows:

  developing the technical design of the IWSES plant, including the transmission evacuation plan

  developing use cases for integrated energy storage appropriate for the Indian system

  sizing energy storage to provide multiple plant and system-level services

  performing cost-benefit analyses of IWSES plants under existing and proposed regulations

  preparing high-level environmental and social impact analysis guidelines

  outlining the financing plan for the projects using viability gap funding and other incentives to support infrastructure projects that are economically justified but fall short of financial viability

  recommending regulatory and policy changes required to integrate energy storage in India

  conducting a reverse trade mission for Indian regulators and policy makers to learn about the energy storage business in the United States. The steps related to the technical design of the IWSES are summarized in the following sections, and examples from the two project sites are also provided.  

Resource Assessment and Siting of Wind Turbines

To accurately assess the wind resource at a project site, it is important to obtain on-site measurements of wind speed and other relevant meteorological parameters. The ideal number of measurement locations is site specific. For the two project sites, the monitoring mast data were quality checked, adjusted to the long-term characteristics, and sheared up to the turbine hub height.

 The wind frequency distribution across the site was determined by combining these data with a wind resource grid file. This information was used for turbine siting and energy production estimation. A geographical information system (GIS)-based approach was used to identify the buildable area suitable for siting turbines within the project, based on land use and other constraints. Offsets were applied to various categories of land use to define exclusion areas. Depending on the features within and surrounding the project, the offset categories included property boundaries, residences, roads, transmission lines, wetlands, streams, protected lands, high slopes, or features that might affect the placement of turbines.

 The wind turbines were then optimally placed within the buildable area to maximize energy and minimize wake losses after taking the necessary interturbine spacing into account. The optimization was accomplished using the commercial wind farm design tool Openwind. Upon completion of the optimization, some turbines were resited to improve project construction and interconnection.   

To make better use of space, wind and solar plants can be colocated, provided that conditions for both wind and solar power generation are favorable at the same location.

In the case of an IWSES, there were some additional considerations made regarding turbine placement to optimize the placement of the solar array. This is discussed in the section “Design Considerations for an Integrated Wind-Solar Plant.”  

Resource Assessment and Siting of Solar Arrays

Good quality measurement of irradiance data is required to perform a solar resource assessment. If usable measurements are not available, it is common industry practice to use high-quality satellite-modeled data sets. For the two project sites, satellite-modeled data were used to achieve the lowest resource uncertainty.

 The long-term period of record from the SolarGIS database was used to estimate the long-term resource and create a typical meteorological year (TMY). The TMY represents long-term resource, temperature, and wind speed at the site on an hourly basis during a typical year.

 Based on available land area, setbacks, site conditions, and industry-standard practices, optimal configurations were developed for two PV solar technologies: crystalline and thin film. A number of parameters were considered in the design, such as tilt angle, azimuth, collector length, pitch, and dc–ac ratio. The main objective of the optimization was to maximize the capacity factor and minimize shading losses, while still achieving a cost-effective design that would meet the capacity targets set for the sites.

 The first step in the system design was to optimize the tilt angle to the site latitude. The collector length was sized based on module dimensions and industry-standard mounting approaches. The array azimuth was set to true south to optimize energy production.

 A pitch/shading analysis was conducted to determine the optimal design of the PV configurations prior to energy simulation. The result of this analysis was used to produce a plot of shading loss as a function of pitch (the space between rows). The pitch/shading analysis resulted in groundcover ratios dependent on the latitude. Next, the solar arrays were laid out to optimize land use, minimize intrusion into exclusion zones, and minimize the potential shading loss caused by proximity to the turbines. The solar arrays were set back from all exclusion zones and boundaries with a 10-m (33-ft) buffer zone. The buffer zones provide setbacks from sensitive areas and a means of access to the solar equipment.

 Roads of 10-m width were outlined through and around the solar plants to provide access.

 A dc–ac ratio of 1.3 was used for the configurations. This dc–ac ratio represents an industry standard that is value optimized for current equipment costs to increase energy production and minimize energy cost. The oversized dc array minimizes the losses upstream of the inverter, which allows for greater energy production at low irradiance and increases the ac capacity factor.

 This initial phase took the design to the point where the mutual interaction of the wind and solar equipment required further consideration.  

Design Considerations for an Integrated Wind-Solar Plant

When designing an integrated plant, it is important to ensure that the interaction between the wind and solar plants is captured and taken into consideration. There are two principal types of interactions that must be modeled: 1) the effect of the turbines on the PV arrays and 2) the effect of the PV arrays on the wind-flow field. The Effect of PV Arrays on Wind Turbine Siting At both project locations, the siting of the turbines was carried out first because the wind resource was more variable across the site than the solar resource; therefore, the optimum configuration of wind turbines is more sensitive to their siting than that of the solar array.

 The presence of the PV panels can impact the surface roughness and affect the wind flow through the turbine array.

 An array of solar panels can be similar to other topographic or locational features (trees, buildings, etc.), in that it alters the surface roughness and affects wind flow through the array. Increased surface roughness changes the profile of the atmospheric boundary layer as it flows across the array, increasing the shear effect.

 The approach for roughness modeling, turbine wakes, and the impact on wind flow is based on a theory advanced by Sten Frandsen (see “For Further Reading”) that defines wind farm equivalent roughness. Frandsen stipulates that an infinite array of wind turbines is represented as a region of uniform high-surface roughness.

 The roughness imposes drag on the atmosphere, causing both a downstream change in the structure of the boundary layer and a reduction in the free-stream wind speed at the turbine hub height. Once the equivalent roughness is defined, it is possible to calculate the hub-height wind speed deep within the array, where the boundary layer has reached equilibrium with the array roughness. At the project sites, the effect was comparable to that of the local vegetation. The wind-flow field model considered the local roughness, assuming that the panels would closely resemble the roughness of local vegetation to account for this effect.

 The difference in the comparative hub height of wind turbines and PV arrays generally yields a small roughness effect on the wind turbines by the arrays. Solar panels may have a significant impact on local roughness if larger foliage (e.g., tall trees) will likely be removed to make room for solar arrays or if a significant area upwind of the turbines in the primary wind direction is expected to be covered with solar arrays.

 However, this type of extreme scenario did not apply for this project. The Effect of Wind Turbines on PV Siting The most obvious way that solar arrays can be impacted by the location of wind turbines comes from the shadow that they cast.

 This shading effect depends on the time of day and the orientation of the turbines with respect to the panels. Shadowflicker analysis was adapted to assess the level of shading impact at receptor locations across the solar arrays.

 Using this customized methodology, the shading fraction at each receptor was calculated for every hour. The affected equivalent array time was computed as a function of the distance between turbines and panels, the wind direction affecting turbine yaw, and the sun’s local zenith and azimuth angle.

 The affected equivalent array time was computed to give a turbine-shading loss estimate. Site-specific solar resource data were used to estimate the solar energy loss. The turbineshading energy loss also accounts for diffuse light that is still present during shading at any time of the day, even though the direct component may be blocked by the turbine.

 This means that turbine shading does not result in a total loss but only a power reduction during array-affected times.

 Due to the electrical effect of modules in series and the large reach of a single wind turbine blade shadow, the analysis assumed that a shaded reflector corresponded to the presence of diffuse irradiance on only that portion of the array for the hourly fraction represented by the model.

 The analysis further assumed that the reduction in incident irradiation on the solar array was proportional to the energy loss expected for turbine shading. Energy modeling was then used to calculate the impact of the turbine shading on the solar production.

 Partly because the effect of the shadows was most pronounced at times of relatively low solar production, it was found that the shading effect of the turbines was lower than 1% of annual production.

 Diffuse irradiation was still present even during times of heavy shading. Optimal Wind and Solar Configuration It possible to arrive at an optimal configuration for both the turbine and solar array layouts only after 1) all site constraints have been defined and met, 2) the time-dependent mutual interactions of the turbines and panels have been built into the energy calculations, and 3) the wind and solar resources have been accurately captured and modeled.

 The wind and solar capacity for each phase of the project was based on the resource assessment.

 An additional levelized cost analysis was performed to determine the inverter configuration as well as the make and model of the PV panels and wind turbines to be employed for each phase. Table 1 shows the wind and solar capacity by project phase  

Design of the BESS

Development of Energy Storage Use Cases The first step in the BESS design process was to identify the uses of energy storage. Based on a survey of the needs of the system as well as the expected revenues in the near and long term, seven applications were identified to be of value. These were then grouped into primary and secondary applications. The primary applications are those for which there is an immediate need for storage or a payment mechanism exists. The secondary applications are those for which a BESS will add value to the system but this value can only be realized based on changes to existing regulations. Primary Applications

 ✔ Penalty charge management under the deviation settlement mechanism (DSM): The first primary application of energy storage addresses financial penalty reduction in the DSM. In India, each state has a state load dispatch center (SLDC) responsible for planning the operation of internal generation to meet the forecasted load. The SLDCs also coordinate the scheduling of interstate generators (generators designated to meet the load of more than one state) with the help of the regional load dispatch center. Any deviations from day-ahead interstate schedules are determined on a 15-min basis, and penalties and incentives associated with these deviations are determined according to the system condition (frequency) prevailing at that time of deviation. In general, when the frequency is below 50.05 Hz and if the state underproduces or overdraws, there is a penalty. Conversely, when the frequency is below 50.05 Hz and if the state overproduces or underdraws, there is an incentive. Figure 3 shows how a BESS can be discharged during periods when the frequency is below 50.5 Hz and the state is withdrawing more than the scheduled energy, as well as how it can be charged during other periods to reduce the DSM penalty.

  Shifting renewable energy to peak demand hours: Another primary application of energy storage is shifting renewable energy to peak demand hours. This application was identified as key for the state of Andhra Pradesh, which has a high target (8,100 MW of wind and 4,081 MW of solar) for renewable energy. During off-peak load hours (8:00 a.m.–2:00 p.m.), the BESS charges and stores energy, which it discharges during peak load hours (6:00–10:00 p.m.). Thus, the integrated plant supplies a consistent amount of energy to customers during peak hours through a combination of wind, solar, and energy storage.

  Forecast deviation reduction for wind and solar plants: Another of the primary applications energy storage can provide is forecast deviation reduction for wind and solar plants. The concept is that energy storage can be dispatched to reduce a renewable energy plant’s forecast errors and thus prevent lost revenues. In India, there is a tiered penalty structure for deviations in wind and solar generation. For example, if the wind generation is below the scheduled value in a 15-min block by 25%, then a deviation charge applies: 110% of the fixed rate for balance energy beyond 15% and up to 25% (i.e., there is no deviation charge for the first 15%).   

Secondary Applications

The secondary use cases of the BESS include 1) ramp management of the integrated wind-solar plant, 2) fast frequency response, 3) primary frequency response, and 4) ramp management for the system during morning and afternoon ramps.

 Sizing of the BESS The generic BESS design process is depicted in Figures 4–6.

 Figure 4 illustrates the first step in the evaluation, during which application-specific battery power command is developed. As an example, from a native wind generation profile and forecast, application-specific controls would generate the storage power needed to bring the total (wind and storage) generation closer to the forecast. By combining these profiles with local market rules and prices, revenue streams (or avoided penalties) can be calculated for the specific application and market.

 Next, as shown in Figure 5, energy storage technologies are selected and sized by processing the battery power command into a use-intensity map. Relying on a database of physics-based performance models, the installed cost to meet the application requirements is calculated for each technology, including storage and BOP.  

An integrated wind-solar plant also has the potential for savings in evacuation and transmission upgrade costs.

Finally, as shown in Figure 6, the asset life is determined from the duty profile, asset type, and size by using asset-specific calendar and cycle-life models. Combining all calculations, project economics can be estimated based on revenue, initial cost, and life (number of replacements over the project life).

 This analysis used a database of energy storage technologies built at GE Global Research for the comparison and evaluation of various energy storage assets available in today’s market.

 The underlying asset database includes equivalent circuit models of over 100 energy storage assets spanning a wide range of technologies and options. These energy storage equivalent-circuit models were developed from a combination of data sheets and follow-up discussions with suppliers to confirm model results. For the primary and secondary use cases described previously, the key input for determining the charge/discharge profile of the BESS was the simulated 15-min wind and solar profiles and the corresponding short-term forecasts for each phase of the project.

 Other key model inputs include wind  and solar tariffs, the DSM penalty structure, and ancillary service tariffs. Based on the technical design and financial analysis, a 10-MW, 15-MWh battery lithium manganese cobalt-oxide BESS was recommended for the phase 1-a 41-MW integrated wind-solar project. This BESS was designed primarily to perform the DSM penalty reduction function but, when not doing so, could also be used for other primary and secondary applications.

 BESS sizing for the remaining phases of the project will depend significantly on future energy storage-related regulations and the cost of the BESS. For each of the remaining phases of the Ananthpur and Kutch projects, the BESS ranged in size from 10–60 MW, with a capacity of 30–280 MWh. Design of the Collector System The collector system was designed using a mixture of industry best practices for wind and PV collector systems. It is common practice in India to use overhead conductors for standalone wind plants and underground cables for PV solar plants. A main consideration in the collector design was  

In an IWSES plant, wind turbines, photovoltaic solar arrays, and a battery energy storage system are integrated into a single plant using state-of-the-art controls. 

whether underground cables should be used for the collector system or a mixed-construction approach using both underground cables and overhead conductors would be preferable. Designs for both cases were compared to evaluate these options. For the mixed-construction case, underground cable was used to feed the PV systems or whenever the conductor path entered a wind turbine exclusion zone.

 In this case, riser poles were used to transition between underground and overhead construction.

 All underground design was used for phase 1-a of the project to account for the tight arrangement of PV arrays and wind turbine exclusion zones, even though the cost was slightly higher than with the mixed-construction approach. For phases with overhead transmission lines, the routing was designed such that there was no loss of PV generation due to shadowing from the lines.

 To evacuate the power, a 33-KV/220-KV pooling substation was designed for each phase of the project.

 The BESS was sited at the pooling substation and connected to the low-voltage side of the substation.  

 The BESS was sited at the pooling substation and connected to the low-voltage side of the substation.  

Design of Controls and Communication System

The final design step was the design of the controls and communications system that enables the BESS to communicate with the plant’s wind turbines and solar arrays and also enables the SLDC and Renewable Energy Management Center to perform the various applications.

 The proposed plant control architecture consists of three control device types:

  Plant energy management system: This system orchestrates all control and communication actions and provides the interface to the utility and SLDC.

 ✔ Plant master control station: This station is a hardened computer that hosts critical real-time control services. Its main function is to compute the set point for the storage system’s power, i.e. the command for the BESS charging or discharging cycles. It also computes the plant-level commands for the wind and solar plants.

 ✔ Plant edge control station: The edge plant-level controllers are hardened industrial controllers located at the BESS as well as at each wind or solar plant. These units control the actual power flow at the point of interconnection based on local measurements and commands received from the master-level controller. The plant edge controllers are typically multifunction controllers that provide the physical interface to the lower inverter-level controllers and sensors.  

Next Steps for the IWSES Project in India

The next step is to develop the 41-MW IWSES plant as a grant-funded pilot project. The lessons learned from the demonstration project can be used to develop the technical design, commercial arrangements, and energy storagerelated policies and regulations that will have a strong influence on future energy storage projects in India.  

We acknowledge the support provided by the U.S. Trade and
Development Agency (USTDA) and Raj Budhavarapu [formerly with the Infrastructure Leasing & Financial Services
(IL&FS) Energy Development Company, Ltd.] in the execution of this study.

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