Solar & Emergency Power FAQ

Understanding Solar Photovoltaic’s Potential for Emergency Power

The City University of New York (CUNY) serves as one of NYC’s prime partners in its coastal storm shelter operations. Sustainable CUNY leads the implementation of Federal, State and City initiatives aimed at creating a comprehensive and streamlined infrastructure for the wide-scale adoption of solar technology in NYC. Together with NYSERDA, Consolidated Edison and leading inverter companies, Sustainable CUNY developed this FAQ document to provide responses to questions raised concerning the potential for solar photovoltaics (PV) to provide power during wide spread utility outages. In the aftermath of Hurricane Sandy it was determined that while the 672 solar arrays on NYC rooftops at that time sustained little or no damage during the storm, they were unable to supply critically needed power during the subsequent outage. While the capability exists, in order to tap into this power resource on a broad scale key issues such as system design, costs, technology integration, incentive structure, codes and regulations need to be addressed. As a result, the City University of New York formed the Smart Distributed Generation (DG) Hub in January of 2013 with Utility, City, State and Federal participation. In additiona, CUNY convened NYC’s first Resilient Inverter Workshop on June 5th, bringing in  five of the top inverter companies to present to NYC solar installers and stakeholders new- or new to New York- technologies that will ideally help create a more resilient New York City.

The following questions and answers broadly address whether it is currently possible to retrofit or purchase a PV system that will provide backup power in the event of grid failure. In answering these questions there are technical, regulatory, and economic elements to consider as well as what would be considered best-practices for critical infrastructure like shelters and hospitals versus homes, multi-family units and commercial businesses.

This guidance document is provided for educational purposes only and does not constitute design, legal, tax or regulatory advice. Please consult with a Certified Energy Manager or Professional Engineer regarding your specific emergency power needs. This document will be continually updated as this segment of the industry is rapidly evolving. Please send comments to nycsolar@mail.cuny.edu

Q1. Why are the current solar photovoltaic (PV) systems in operation in NYC unable to provide backup power during power outages?

Q2. Can the PV systems currently in operation be retrofitted to provide backup power?

Q3. Are there commercially ready PV systems currently available that can provide (some) backup power?

Q4. Is it allowable to have a net-metered PV system that automatically switches to battery backup in the event of grid failure (with the PV charging the batteries directly for the duration of the grid outage)?

Q5. Can this kind of system be installed by any installer, or do only a limited number of them offer it and have the necessary technical ability?

Q6: What are the typical costs for these systems?

Q7. Are there any incentives?

Q8. Are there additional benefits besides supplying emergency power- to support investing in energy storage or hybrid systems? Are there additional financial benefits of a solar PV with battery system (e.g. peak demand shaving, storage for onsite use based on time-of-day/use pricing, ancillary services, etc.)?

Q9. What are the approximate additional lifetime costs of such a system (e.g. battery and inverter replacement)?
 

Q10. Are there only certain types of Systems available?

Q11. What additional operating conditions should I consider when specifying or designing a PV system with battery storage (and hybrid system with a generator)?

 


 

 

Q1. Why are the current solar photovoltaic (PV) systems in operation in NYC unable to provide backup power during power outages?

Q1 A1. In order for your PV system to be interconnected to the grid, your system must comply with the New York State Standardized Interconnection Requirements (NY SIR) as created by the New York State Public Service Commission, along with agency regulations such as fire, building and electric codes. To ensure the safety of the public, utility personnel and grid equipment, your grid connected PV system must power down in the event of a grid failure. Specifically, the SIR states that “The generator-owner shall not supply power to the utility during any outages of the utility system that serves the PCC (Point of Common Coupling). The generator-owner’s generation may be operated during such outages only with an open tie to the utility. Islanding will not be permitted. The generator-owner shall not energize a de-energized utility circuit for any reason (NY SIR, April 2012). Likewise the Point of Common Coupling (PCC) is defined as “The point at which the interconnection between the electric utility and the customer interface occurs. Typically, this is the customer side of the utility revenue meter”. www.dps.ny.gov/distgen.htm.

 


Q2. Can the PV systems currently in operation be retrofitted to provide backup power?

 

Q2 A1. Yes, by working with an energy auditing company and/or your solar developer, you would need to determine your emergency power needs; then determine the commercially available system that meets your requirements. Q3 provides a breakdown of the typical options.


Q3. Are there commercially ready PV systems currently available that can provide (some) backup power?

Q3 A1. A first option to consider is to have a “daylight emergency power” (power when the sun is shining) single emergency outlet installed utilizing a PV inverter that offers this option. Existing systems can be retrofitted with this option. During a power outage when your inverter automatically shuts downs to avoid feeding power to the gird, you can manually enable a 12amp (125V) single plug outlet that allows you to use small applications like a cell phone charger or laptop, again, only while the sun is shining. It is therefore limited to ‘real-time’ use because there are no batteries and the inverter that powers your single outlet operates in a voltage source mode that must still, for instance, accommodate for clouds causing solar power to be intermittent. This system is the lowest cost option with a minimal incremental difference to standard inverters.
Costs range from $0.30/W to $0.50/W – see Q6.

Q3 A2.  Another option is to step up to a technology-leading inverter that is capable of both on-grid and off-grid operation, with or without a battery. Without a battery, the inverter can power a small number of power outlets (eg 5 outlets), or power a larger variable load like a water pump or a fan, when sunlight is available. This solution is limited to real-time use because there are no batteries and the inverter that powers your outlets or variable loads operates in a voltage source mode that must still, for instance, accommodate for clouds causing solar power to be intermittent. These inverters are generally referred to as bi-directional and allows for the incorporation of batteries during installation or in the future. See Questions 11 & 12 (Q11, Q12).
Costs range from $0.30/W to $0.50/W – see Q6.

Q3 A3. You can purchase a grid-connected PV system with backup off-grid capabilities that includes a battery. Typically this is done by connecting your PV system to the grid via your main inverter, and installing an additional off-grid inverter that is connected to a battery system for emergency operation. In the event of a grid power outage, your main inverter would disconnect your system from the grid; then your off-grid inverter would allow you to use the energy from the batteries and could recharge the batteries when solar resources are available. Typically this battery system only powers ‘critical’ equipment. Please note that there are other energy storage options, such as hydrogen via a fuel cell. Depending on duration of energy storage required before re-charging, this option might be more expensive than a hybrid system discussed in a4 below.
Costs range from $0.30/W to several dollars per Watt primarily depending on the system size relative to your emergency power needs for the duration of the emergency power event – see Q6.

Q3 A4.  An additional option is a hybrid system that includes a gas or diesel generator or another distributed generation technology that is integrated with a solar and a battery system. Please note that typical residential generators do not generally meet the power quality requirements for synchronizing the PV inverter. Your solar developer will need to comprehensively specify and design the PV, inverter, generator, charge controller and batteries for optimal equipment life and sufficient emergency power quality similar to most commercial systems. A hybrid system would generally allow for a smaller battery size and thus might be more cost effective than a PV and battery only system.
Costs range from $0.30/W to several dollars per Watt primarily depending on the system size relative to your emergency power needs, along with the generator operations, maintenance and fuel costs, for the duration of the emergency power event – see Q6.

 


Q4: Is it allowable to have a net-metered PV system that automatically switches to battery backup in the event of grid failure (with the PV charging the batteries directly for the duration of the grid outage)?

Q4 A1.  A PV system cans net-meter under New York State Standardized Interconnection Requirements (NY SIR). However, based on the current NY SIR rules, batteries are not an allowable technology for net-metering (emergency or otherwise): www.dps.ny.gov/distgen.htm.

Q4 A2.  The PV system could operate in an emergency mode as long as it satisfies the current SIR rules along with local regulations. Thus, in emergency mode it is disconnected from the grid and by definition cannot net-meter. In the event of a grid power interruption, your PV system’s point of common coupling (PCC) would need to be open, then your PV system could charge batteries and support your energy needs through your critical loads sub-panel. Three examples of this type of operation were discussed in Question 3 (Q3).  It is important to also confirm that your grid connected inverter is UL 1741 certified and thus eligible for interconnection (see pg. 29 of the 2012 NY SIR: www.dps.ny.gov/distgen.htm).

 


Q5. Can this kind of system be installed by any installer, or do only a limited number of them offer it and have the necessary technical ability?

Q5 A1. Most installers do not have extensive experience in emergency power system specification and design due to the limited market for these systems in the past. However, no additional certifications are required and once trained, they could deploy systems. CUNY is currently coordinating with inverter companies to offer workshops for installers in the coming months.

 


Q6: What are the typical costs for these systems?

Q6 A1. As per Q3 above, there are a few options for deployment that in-turn increase system costs. At the lowest capability level, an inverter that allows for limited off-grid operation ranges may cost from $0.30/W to $0.50/W, and at the lowest level of functionality would provide limited power when it is sunny for charging cell phones or laptops. While still only available with sunlight, inverters are available that can power a small number (e.g. 5) of power outlets, or power a larger variable load like a water pump or a fan.  For battery system integration, the costs range from $0.30/W to several dollars per Watt. Costs depend on battery type and size, duration of storage, frequency of discharge, and charge controller quality, as well as wiring/rewiring for implementing a sub-panel breaker-box with only critical emergency loads that might also employ load management schemes. An additional criterion is that for a hybrid system there are multiple distributed generation technologies and fuels (e.g. fuel cells using natural gas/hydrogen/biofuels, solar, wind, and multiple storage technologies, thus integrative system design and analysis is required.  See Q11

 


Q7. Are there any incentives?

Q7 A1. The policy environment for energy storage and/or emergency power is rapidly evolving, but there are limited incentives at this time. Battery systems are generally not eligible for the Federal Investment Tax Credit (ITC) that is typically employed with solar financing, but this does depend on the inverter design. For example, bi-directional inverters can support both the batteries and the PV, and thus would be eligible for the ITC. For additional information see DOE’s energy storage database: http://www.energystorageexchange.org/policies

 


Q8. Are there additional benefits besides supplying emergency power- to support investing in energy storage or hybrid systems? Are there additional financial benefits of a solar PV with battery system (e.g. peak demand shaving, storage for onsite use based on time-of-day/use pricing, ancillary services, etc.)?

Q8 A1. An integrative designed and deployed system could be used for additional energy services such as:

•    Peak shaving and demand reduction;
•    Reducing energy bills based on time-of-use/day pricing;
•    Ancillary services (e.g. voltage/frequency control) if applicable

Likewise, designing for Uninterruptable Power Supply (UPS) versus Emergency Power Supply drives from different power quality needs and durations. The specific benefits for a customer vary, based on:

•    System design
•    Utility rate structure
•    Applicable services captured by your system
•    Market segment (residential vs. commercial) and required power quality.

The available incentives are evolving with the technologies and therefore should be evaluated with your solar development company or an energy services company.

 


Q9. What are the approximate additional lifetime costs of such a system (e.g. battery and inverter replacement)?

Q9 A1.  Lifetime costs for a battery system depend on your battery type, duration of storage, frequency of discharge, dispatch strategy and lifetime replacement of your batteries and power conversion equipment (inverter and charge controller). Battery lifetimes are affected by battery chemistry as well as dispatch parameters, such as depth of discharge, rate of discharge, discharge duty cycle and operating conditions (temperature, humidity, etc). Lifetime performance ranges from less than one year to sometimes beyond 10 years when infrequently and lightly used. Thus lifetime costs occur during the replacement year, as well as proper disposal of the old equipment. Operations, lifetime and warranty specifications will be needed during system design and analysis so as to include these costs. Please note that similar to the PV industry, battery markets are rapidly expanding as their cost come down while reliability and lifetime improve.

Q9 A2. Inverter replacement would correlate with what is typical in the PV industry, varying from 10 to 20 years. Operations, lifetime and warranty specifications will be needed during system design and analysis to properly account for these costs.   

 


Q10. Are there only certain types of Systems available?

Q10 A1. The most mature and market-deployable systems currently rely on PV, the inverter options mentioned in Q3 above, and deep-cycle lead-acid batteries. There are tens-of-thousands of these systems implemented across the country. There are many distributed generation technologies, such as solar, wind, gas/diesel/biofuel generators, microturbines, co-generation, fuel-cells, and tidal & wave systems. Likewise there are several energy storage technologies, such as advanced lead-acid batteries (2000+ cycle life), sodium/sulfur batteries, lead-acid batteries with carbon-enhanced electrodes, zinc/bromine batteries, vanadium redox batteries, lithium-ion batteries, hydrogen storage combined with a fuel-cell, compressed air energy storage, pumped hydro, flywheels, supercapacitors, and more.

 


Q11. What additional operating conditions should I consider when specifying or designing a PV system with battery storage (and hybrid system with a generator)?


Q11 A1. You will need to determine the duration and nature of your potential emergency power event, such as:

•    Number of day of self-generation you will need to design for, under various circumstances
•    The critical loads and their priority that you will need to power in the event of an outage

Q11 A2. For a hybrid system, your design must consider:

•    Loads
•    Generator
•    PV size in an integrative manner
•    Operation under low load situations or
•    Operation under high frequency

Variation of solar generation must be considered in terms of power quality, system cost and system safety. Consider:

•    The conditions under which you would ‘load shed’ if needed
•    Your priorities
•    Equipment safety under low load
•    A design that prevents PV power back feeding to the generator in order to avoid damage to the control equipment

Q11 A3.  You will need to evaluate the UL listing of your power conversion and power generation equipment to ensure your power quality specifications meet your energy load requirements as well as each other’s:

•    Voltage
•    Current
•    Surge
•    Reactive power and harmonic

E.g. when designing a hybrid system you will need a generator that produces a quality phase signal sufficient for your PV inverter synchronization, as well as load matching and control to ensure your generation matches your load.

Q11 A4. Other considerations include software and communication technologies that will be needed to be implemented for your level of system communication and control. For example:

•     Customer notification/control application for your cell phone
•    Utility SCADA communication/control

 


This document will be continually updated as this segment of the industry is rapidly evolving. Please send comments to nycsolar@mail.cuny.edu