Solar Microgrid at Work Lessons learned from completing one of the first commercial scale grid-interactive solar microgrids in the US By C.J. Colavito, REP
Introduction Standard Solar, Inc. recently completed one of the first solar microgrid systems with a gridinteractive battery bank in the country. Being a first was a challenge it took months of dedication, innovative engineering and coordination with key partners, utilities and government offices to make this project a reality. The first half of this paper will set the stage by explaining how the microgrid is setup, its functionality and what makes it special. Then I will explore what it takes to design and install a solar microgrid system, the lessons learned from this groundbreaking project and what technical considerations should be made when implementing this new technology. Project Summary The Konterra Solar Microgrid project is the product of cooperation between Standard Solar, Solar Grid Storage, the Maryland Energy Administration (MEA), and Konterra. The project consists of a solar PV parking canopy DC coupled with a Lithium Ion energy storage system using two dual function 250 kw inverters at the Konterra headquarters building in Laurel, MD. This project is unique This project is unique because both the PV system and the energy storage system are grid interactive. because both the PV system and the energy storage system are grid interactive. Konterra is a real estate development and management company who is the PV system owner, host, and energy off-taker. Standard Solar developed the project and prepared a grant proposal to the MEA for a Game Changer grant to support this cutting edge project. Our partner, Solar Grid Storage, contributed as the financier, designer, integrator, and operator of the lithium ion energy storage system and dual function inverters. Standard Solar also performed all engineering, procurement and construction services for the integration and installation of the solar microgrid system.
Project Partnership Structure Project host Owns solar system (not inverters) Received all tax benefits (ITC) Solar production offsets electricity from Grid Developed Project Prime contract with Konterra to design & build system Submitted and received MEA grant Project Engineering and Construction Operations and Maintenance Awarded competitive $250k grant Supported Game Changer innovation, development and implementation Owns batteries and inverters Receives revenue for providing frequency response services to grid (PJM) Subcontractor to Standard Solar for a portion of the installation There are a few important technical and financial details that make this project unique from other renewable energy microgrids or a simple solar PV system with battery back-up. First, the energy storage system is grid interactive. It is designed primarily to provide the PJM regional transmission grid with fast response frequency regulation support. According to PJM, Regulation service corrects for short-term changes in electricity use that might affect the stability of the power system. It helps match generation and load and adjusts generation output to maintain the desired frequency. Participation in the frequency regulation market allows the storage system to generate significant revenue for the energy storage system owner. This revenue cannot be captured by a battery system that is used exclusively for emergency back-up power. The solar PV system and the battery system are DC coupled, meaning that they share the same inverter. In addition to the frequency regulation revenue, the storage system is able to take advantage The energy storage system is designed to primarily provide the PJM regional transmission grid with fast response frequency regulation support. of the federal investment tax credit since it is DC coupled with the PV and several of its components are essential to the operation of the PV system, such as the inverter. Additionally, because the inverter is shared by the PV and the battery system, the PV system does not require the purchase of a separate inverter. This results in a reduced purchase price for the PV system. All three of these financial advantages combine to enable a cost effective means for the project
host to have access to the battery system and the PV system as a source of back-up energy supply. Functions & Capabilities The solar microgrid system is designed to operate in two modes; Grid-Interactive and Islanded mode. In grid-interactive mode the battery system operates in parallel with the PV system. The PV system operates normally as a typical grid-tied solar PV system. During peak sun hours of the day the battery system is less active, but when the PV system is not utilizing the majority of the inverter capacity (i.e. at night) it is able to actively participate in fast response frequency regulation. The control system is designed to always prioritize the use of the inverter capacity for the solar PV generation first, then the remainder is utilized for frequency regulation participation. In full sun the PV system will normally require approximately 325 kw of AC capacity, leaving 175 kw of inversion capacity available for participation in the frequency regulation market. When there is a grid outage the microgrid system senses the loss of grid and signals the isolation breaker to open and convert to Islanded mode. The system adjusts automatically from a grid-tied current source to an islanded voltage source in a few cycles. The PV system will continue to produce electricity as long as there is sufficient sunlight to generate and sufficient load or battery capacity to absorb it. The energy storage system acts as a buffer between the PV and the load so that the user doesn t notice any fluctuation in power as a result of unstable sky conditions. The duration that the energy supply will last is difficult to predict because it is a function of the amount of sunlight available, the demand of the selected back-up loads and the state of charge of the battery system at the moment of isolation from the grid. In order to properly and safely operate and automatically switch between the two modes explained above a comprehensive monitoring and controls system was installed along with several critical pieces of integration equipment. In order for the microgrid controls system to see what is happening at each point in the system several power meters were deployed at key points in the system. A total of 8 shark meters were installed; one the PV DC input to each inverter, on each battery DC input to each inverter, one at each inverter AC output, one on the main busbar of the back-up load center and one meter measuring the net load of the building. All of these meters allow the controls system to monitor what is happening at all vital points in the system in any operation mode and provides closed loop verification of switch b. critical load subpanel with automatic isolation breaker a. Critical load subpanel with automatic isolation breaker operation. Along with the metering, a new 1600 Amp load center was installed to accumulate the AC outputs of the two inverters and to function as a critical loads sub panel for backed-up circuits. The main breaker of the new critical load sub panel is a Siemens Sentron 1200 Amp remotely operable molded case circuit breaker. This breaker is the isolation breaker for the system to island itself from the grid before continuing to operate during a grid outage. The last main piece of equipment used for the integration is a 1200 Amp enclosed circuit breaker with
ground fault detection and interruption capability. This device acts as the first over current protection device in the system after the supply side busbar tap in the metering section of the building s main switchgear. In order to meet NEC code requirements the switch was required to be service-rated, have ground fault detection and interruption. Additionally, a concrete encased duct bank was required between the tap and the breaker. Technical Equipment Details 402.3 kw Solar PV Parking Canopy System Suniva American Made PV modules Solaire 360T Parking Canopy System SolarBOS disconnecting combiner boxes Locus Energy Monitoring System with public view lobby kiosk Two level 2 Sema Connect EVSEs, with infrastructure for four additional EVSEs Containerized Battery Storage System Two 40-foot containers each with: Princeton Power 250 kw BIGI inverter All Cell Li Ion Battery bank SolarBOS contactor recombiners with DC zone level current sensing Monitoring, Controls and HVAC Equipment 500 kw total Inverter capacity 300 kwh total storage capacity
Technical Considerations and Lessons Learned Throughout the process of design, construction, and commissioning the project team worked through many challenges and obstacles to completing this unique microgrid system. The next section is a compilation of key lessons learned. It also delves into important technical considerations that need to be taken into account when implementing this type of solar microgrid in the future. I have divided up the points into three sections; Engineering and Design, Project Management and Logistics, and Safety. Engineering and Design Considerations One of the critical items that was taken lightly early on in the project is the Sequence of Operations (SOO). This is the document that clearly explains the various operation modes of the microgrid system, indicates what each piece of equipment is intended to be doing and what switches, components or alarms should be on in each mode. It was assumed that the SOO was a standard document that comes with the high tech dual port inverters which have to manage both the energy storage and PV systems simultaneously on the DC side. However, we learned that the SOO is somewhat custom for each job and it can be highly dependent on how the system is integrated into the building, what other components or energy sources are present (i.e. a generator) and what the design intent of the system is. Therefore it is essential that the SOO be customized for each project and be discussed with all key stakeholders, such as, the host client, the inverter manufacturer, engineer of record, battery system operator, etc. The SOO needs to be comprehensive and it is important to run detailed what-if scenarios and worst-case scenarios when developing the SOO to ensure all modes are accounted for and controls system will function as needed for any case. It is also important to consider building-in redundancy and closed loop verification for critical data points and controls operations, for example, the isolation breaker position. A well-developed SOO will make it easy to determine where critical points in the system are that need instrumentation or controls. There are a lot of practical design details that need to be accounted for in advance of material orders and site mobilization. Consider the location of the inverters, PV array, battery bank, point of interconnection, and critical load back up panel at the first stage of design. Think about where each of the major system components are relative to one another and how all the connections will be made. Take into account the costs and design implications of where equipment is located. Carefully evaluate options for storage system container placement. It is important to think about accessibility, practicality, proximity to the rest of the system as well as constructability. Integration of the critical load back-up panel can also be challenging. Each building is unique and requires a custom design for the interconnection and layout of the critical load panel, main service disconnect, and grid isolation device. In addition to equipment placement it is important to think about the total and net current and power flow at various points and directions throughout the microgrid system. One easily missed detail is to consider the bi-directional capacity of the host customer s utility transformer. A 500 kw AC PV system is not a problem exporting PV energy all day into a 750 kva utility
transformer. However, if it has energy storage and is in charge mode drawing full power, the transformer could easily be overloaded if the host is drawing more than the balance of the transformer capacity. Along with evaluating the microgrid systems impact on the service transformer the critical loads selected for backup are also important to assess. Some loads may not operate well on the microgrid depending on their current draw and the inverter capacity. Sometimes large inductive loads with high in-rush current requirements will not function properly on a microgrid driven by a PV inverter. Code Review National Electric Code NEC 110.26 Working space requirements NEC 230.95 Ground fault protection required for services rated 1000 A or more NEC 230.7 Service disconnecting means location NEC 230.6 Concrete encased service conductors NFPA 70E Arc Flash Hazards International Building Code OSHA When preparing the pro forma for a solar microgrid project it is important to understand that the PV system will not necessarily deliver the same amount of electricity to the grid as a standard grid tie system would. When running the production model there are some key items to consider. First, the dual function inverters available today are not typically capable of the same high conversion efficiencies we get from the best grid-tied inverters. Storage inverters run at efficiencies between 92% and 96%, where the best grid tied inverters can reach 98.5% peak efficiency. Container electrical loads also need to be accounted for. Most battery storage systems require space conditioning to maintain battery cycle life and safety. Throughout a year the HVAC loads of the storage system container add up. In addition to the HVAC loads it is important to take into account the round trip efficiency of the energy storage system s charge/discharge cycle. Most high quality Li Ion systems have a 90% round trip efficiency, but it is dependent on the battery chemistry and inverter efficiency, among other factors. The losses associated with losing 10% of each kwh that is absorbed by the battery bank can easily result in thousands of dollars a year. Lastly, depending on where the renewable energy credit meter is located some of these losses can detract from the number of SRECs earned by the PV system. This can be an important point, which affects the entire design approach. The figure below compares the pros and cons of DC vs. AC coupled energy storage systems with solar PV. Using
an AC coupled design approach can mitigate some of the issues noted above, but also comes with some drawbacks. Pros and Cons of DC Coupled vs. AC Coupled Energy Storage systems DC Coupled AC Coupled Pros Cons Pros Cons Only purchase one inverter to operate batteries and PV May improve battery charging efficiency Less space required, less redundant controls and equipment Allows for more intelligent coordination between PV and battery because it is all internal to the inverter. More easily obtain qualification for Federal ITC on storage system and components Inverter conversion efficiency is lower Very limited on choices for inverter equipment New approach and that is less understood by manufacturers and integrators PV SRECs earned can be reduced AC side maintenance requires shut down of storage and PV PV electricity generation output metering on AC side cannot be measured independently of storage. Storage system and inverter don t need to communicate directly with PV inverter, simply read AC output data. More common approach, many choices for listed equipment providers No conversion efficiency loss on PV AC side No loss of SREC value on PV generation Easier PV performance verification and measurement Easier isolation of PV and storage system for maintenance and trouble shooting Can be more expensive Requires more space, components and labor to install Controls are less integrated between separate inverters Monitoring systems for PV and battery need to be custom or independent from one another. Federal 30% ITC is more challenging to support Project Management and Logistics Considerations Although there are less unusual details that must be overseen in the project management phase as compared to a typical grid-tied PV project, they can be equally as important as the design lessons. It starts with communication and managing expectations of the host customer, vendors, electric utility and the inspector. For the host customer it is critical that they are fully educated in the functionality and capabilities of their solar microgrid. Consider the total load of the backed up circuits, expected load behavior and ability of the user to adjust consumption as needed during an islanded operation with limited energy storage. Guide the host customer on the operation of the system in islanded mode and how to manage their loads to get maximum run time with minimum impact on their operations.
Communication is also key with equipment vendors. Microgrids are new and unusual applications. Work closely with all equipment manufacturers on the expected operation of their components and how they will fit into and operate in the system. Validate communication protocols and compatibility with the control system. Confirm NRTL listing for all major components, take into account the special application of a grid-interactive micro-grid. Work closely with vendors and make sure that their certification schedules will meet delivery and installation timelines. Logistics of the battery delivery and connection are important. The batteries should not sit idle and unplugged, they need power to maintain charge or could be damaged. In addition to working with vendors, the electric utility could be a stumbling block. A strong working relationship with the utility company is essential for obtaining timely interconnection approval. Begin the process with a face to face meeting to discuss project details with utility engineers and administrators. Lastly, plan ahead to provide for a reliable high speed data connection for communication to the regional transmission operator. Four second interval response is required for fast response frequency regulation signals. A dedicated land line connection is normally needed. If the host s connection is not sufficient the storage assets could be sitting partially idle until a high speed drop is installed. Safety Considerations Safety could have been the first section, but I thought it would be better to save it for last so it could be the most fresh in your mind. With a microgrid system you are essentially operating your own small utility. Just like any other utility it is important to have a thorough understanding of how the system works as well as good documentation along with policies and procedures for normal operations and emergencies. Start by designing in convenient emergency shut-down mechanisms. Safety procedures should be easy to execute and intuitive, not convoluted and unclear. A practical design detail that is important for safety is to make sure that automatic or remotely operable disconnecting and isolation means are available on both the DC and AC sides of the PV array and energy storage system. A simple fused recombiner box for the PV DC input is not a good idea. A DC load break rated contactor recombiner or shunt trip capable circuit breaker recombiner makes it easy to integrate an emergency stop system that isolates the PV from the microgrid with the push of a button. This approach also provides for safer operation of the AC circuit breakers and switches so they can be opened while not under load, even though they are load break rated. This is important for avoiding exposure to arc flash in the event of a breaker or switch malfunction. Also, make sure that an arc flash hazard analysis (AFHA) is conducted. This is especially important since there will be remotely and automatically operated switches. Short circuit and overcurrent protection device c. Inside of a SolarBOS contactor recombiner box
coordination studies go hand in hand with AFHA. Both should be done and any potential issues should be resolve in the engineering phase. A 40 foot long energy storage container full of batteries is inherently dangerous but many of the risks can be mitigated with a few safety features. Include a customized fire suppression system in the container. The All Cell battery packs used for the Konterray microgrid are protected by an individual cell isolating phase change material which absorbs waste heat and eliminates the risk of over-heating, however the batteries aren t the only source of heat and fuel inside. Also consider if it is necessary to integrate the fire alarm system with the host building. Depending on proximity and ease this could be a good choice. d. Inside of one of the 40 foot battery storage system containers Container design is critical verify proper working clearance and that it meets building code requirements. Keep in mind lighting, ingress and egress routes, fire alarm systems, and the use of flame resistant materials. There are a lot of components that need to fit in a limited amount of space. Laying out the equipment and providing for working room can be tricky while maintaining other code requirements, as demonstrated in the photo to the left. Lastly, it is a great idea to hire a third party expert to thoroughly test the system at the time of commissioning. This provides for accountability and allows for one more set of eyes to check it all out before it goes live.
Summary and Key Take-Aways This microgrid project is unique and game changing due to its use of the battery bank for grid support functions and creative financial model to generate revenues, capture incentives, and create savings through the energy storage system. This type innovative model for incorporating battery back-up capability in a cost effective way is already in high demand. Organizations like the Department of Defense, Federal, state, and local governments, hospitals, universities and critical infrastructure are already beginning to require microgrid capability for PV systems they host. The technology is available now and ensuring that these solar microgrid systems are designed and installed properly will accelerate the growth of this technology and the PV industry. I have boiled down my experience with the project to 8 key lessons learned and technical considerations for a solar microgrid: 1. Document a comprehensive Sequence of Operations and communicate it well to all stakeholders 2. Carefully consider equipment layout for practicality, code compliance and cost effectiveness 3. Evaluate total building loads and selected back-up loads at the design phase 4. Account for losses and storage system energy consumption in the pro-forma production model 5. Consider the pros and cons of an AC coupled vs. DC coupled system for your specific application 6. Manage the host customer s expectations of the microgrid and battery system performance based on their selected back-up load behavior and the system design 7. Maintain frequent, clear and pro-active communication with all key stakeholders (host customer, AHJ, utility company, equipment vendors, etc.) 8. Safety Considerations should not be an afterthought; design them into the system upfront. Remember that a microgrid is just a small utility.