Prospectus of CPES Mini Consortium on Renewable Energy and Nanogrids (REN)

Prospectus of CPES Mini Consortium on Renewable Energy and Nanogrids (REN) Fred C. Lee, Dushan Boroyevich, Paolo Mattavelli, Khai Ngo Center for Power Electronics Systems Bradley Department of Electrical and Computer Engineering College of Engineering Virginia Tech Blacksburg, VA August 2010

Mini Consortium: Renewable Energy and Nanogrids Objectives The program Renewable Energy and Nanogrids will concentrate on finding integrative solutions to satisfy the energy, functional, comfort, and zero CO 2 emission goals for building/home environment. To achieve these goals, we will implement a living lab that will provide simulated environment for research, evaluation, and demonstration of advanced technologies for sustainable buildings. Building upon the Center s Sustainable Building Initiative (SBI) sponsored under the NSF ERC Program, with the initial focus on the development and demonstration of advanced power electronics technology for electrical systems in sustainable buildings, CPES will further develop ac and dc based renewable energy powered system as a testbed, a living Lab, for future sustainable building electric power system. The renewable and alternative energy sources would include primarily photovoltaic solar cells, wind generators, micro turbines, fuel cells, and energy storage. The testbed will be used as a vehicle to address many of the nanogrid and grid interface related issues, such as dc bus architecture, energy/power management, and various forms of utility interface converters and inverters. The site of the living lab will be the home of CPES in Whittemore Hall at Virginia Tech, where many of the facilities have already been installed, as shown in Fig.1. Fig. 1: Solar panels and wind turbine installed by CPES atop Whittemore Hall. 1

Sustainable Building Initiative The Sustainable Building Initiative was funded under the NSF ERC Program in 2007, and was driven by the desire to improve energy efficiency in the home and in buildings. Homes and buildings provide one of the largest opportunities for both improving energy efficient utilization and for distributed energy generation. Figure 2 provides a high level schematic illustrating the interconnections and functions of a variety of home electric energy components that together comprise a sustainable home. It is envisioned that through appropriate design of these components and by operating them in a coordinated fashion, net residential fuel based energy use can be reduced dramatically, while simultaneously increasing the perceived comfort levels in terms of lighting, temperature, water and air quality. Fig. 2: Conceptual power electronics based electric power system in future sustainable home. Major features that distinguish the operation of the proposed electrical system in comparison to today s building energy system include the following: Renewable energy generation (e.g., solar systems, wind) Local fuel based energy generation (e.g., micro CHP systems) EV/Plug in hybrid generation/charging/storage Bidirectional connection to the grid that allows energy trading Ability to work in islanded mode and thus ride through most grid outages Responsive illumination control (e.g., LEDs, CFL) 2

Process optimized appliance operation control (air, water, HVAC, ) Sensor network (for appliance, lighting, process energy management). Power electronics plays a major role in the successful implementation of the home energy system shown in Fig. 2. Although not explicitly shown in this figure, the majority of the electrical household functions identified in this figure depend on the availability of compact, highreliability, low cost power electronics to convert electrical power into the form and amplitude required by the source or load. It is expected that much of the power electronics technology developed by CPES will be valuable in order to find innovative engineering solutions for these critical residential applications. The prominent presence of renewable/alternative energy sources in sustainable home architectures emphasizes the fact that achieving the objective of net zero electrical energy consumption from the electric grid provides much of the underpinning for the sustainable home concept. That is, the home is expected to act as a net energy producer delivering electrical power to the grid during the day when the sun is shining and electrical consumption is low due to low occupancy levels while children are in school and parents are working outside the home. However, the home becomes a net electrical energy consumer from the grid during other parts of the day when occupancy is higher and renewable energy sources such as sunshine are not available. An important concept associated with the sustainable home that is not highlighted in the Fig. 2 sketch is the opportunity for individual homes to function as nanogrids. That is, the electrical interconnection between the home and the electric grid can be designed so that the home islands itself to operate as an independent electrical system if electrical power is lost from the grid due to storms, etc. Under these conditions, the electrical system in the home must be designed to transparently assume responsibility for managing the internal sources and loads in order to maintain stable operation of the electrical system until electric grid power again becomes available so that reconnection can be established. The availability of electrical energy storage devices will be important for improving the robustness of such a home nanogrid in the presence of intermittent electrical sources including wind turbines or photovoltaic (PV) panels. Sustainable Building Design Initiative In addition to the NSF ERC support, a 2 year Sustainable Building Design Initiative has been launched since July 2009, with funding support from The Institute for Critical Technology and Applied Science (ICTAS) at Virginia Tech. It is an interdisciplinary effort between the Center for Power Electronics Systems (CPES) and the Interior Design Program in the School of Architecture and Design to redesign the current CPES space by integrating new technologies that would provide an advanced platform for research. To create a living lab environment, CPES at Virginia Tech is modifying a research lab to incorporate emerging and anticipated future home/small office renewable energy technologies and power management systems. Four rooms are being converted into a living lab for students, faculty and staff: a conference room, library/lounge, kitchen, and computer workstation office. Future electrical loads that will be used include: plug in hybrid electric vehicles, high efficiency 3

light emitting diode (LED) lamps, and next generation home appliances, such as a washer, dryer, microwave oven, stove, dishwasher, refrigerator, air conditioners, television, audio systems, and home robotics. The lab will be supplied by the experimental dc bus electrical distribution system with automated source and load management that will be powered by solar and wind generators interconnected with plug in hybrid electric vehicle battery subsystem, and grid. The home/small office will then be both a supplier of energy to the power company and a user when consumption of energy exceeds the locally produced renewable energy. The goal of the Interior Design team is to enhance the quality of the CPES work environment, while providing a versatile space that encourages students to conduct cutting edge research on energy usage, Fig. 3. In addition, select pieces of furniture have been collaboratively designed with an integrated power supply one step forward in an effort to eliminate the need for stationary electrical outlets that limit options for furniture placement and that are quite often aesthetically displeasing. A key component of any spatial design is lighting; therefore, the Interior Design team has carefully designed this space to provide ample lighting and increased lighting controls. Not only will this allow more versatility for its users as the needs of the space changes, but also transforms the space into a living lab where the Interior Design team can conduct tests on perceptions of brightness under fluorescent and LED light sources. Furthermore, the Interior Design team will conduct tests on specified cradle to cradle materials, their durability, and the amount of off gassing of volatile organic compounds (VOC s). Fig. 3: Conceptual design of computer workstation office and kitchen spaces in CPES Living Lab. AC Nanogrid within Contemporary Smart Grid Concepts Contemporary trends and higher availability of smaller generating systems (i.e. solar cells, wind turbines) have opened new opportunities for electricity users to generate power on site. The so called microgrid is widely known and accepted concept that comprises energy storage and a larger number of generating units in order to get the most from the naturally available renewable energy sources. As an example, Fig. 4 shows the microgrid concept as it is being applied to residential electrical systems at low power level: the nanogrid. 4

In the contemporary homes like the one shown, the majority of the electrical household functions depend on the power electronics to convert electrical power into the form and amplitude required by the sources or loads. A modern washer, for example, features the variable speed drive that provides a maximal torque per current to the brushless motor, and thus more efficiently utilizes the energy compared with the conventional pole changing induction motor often found in the older type of washers. Even once the biggest resistive load in a home, a stove, is being replaced in advanced countries with the one featuring induction heating and now becomes an electronic load to the system. Lower power consumer electronics devices as TV, computer, audio and other portable ones, inherently comprise the power electronics circuit for operation, and boast the power aware function enabled by it. Fig. 4: A contemporary vision of smart electrical system nanogrid in residential buildings. In the Smart Grid concept, the Energy Control Center (ECC) in Fig. 4 consists of a smart power meter and remotely operated (disconnect) breaker. ECC is able to communicate with the power system operator for the energy trading purposes, and also acts as data acquisition unit collecting and recording the power flow data not only from/towards the grid, but also from all the converters and smart appliances in the home. The major features that distinguish operation of the shown electrical system in comparison to today s conservative home include the ECC, renewable energy generation, PHEV and/or local battery storage. Power converter for the PV is most commonly unidirectional two stage converter featuring the step up (boost) dc dc 5

converter stage and a voltage source inverter stage for adequate interface with the utility grid. New converters for small wind turbines are also two stage power converters, comprising the active rectifier and a voltage source inverter that serves as ac line interface converter. Energy storage and PHEV typically require the bidirectional ac dc converters for the optimal battery utilization on one side, and ac line interface at the other. Since 2007, CPES has developed and demonstrated unidirectional and bidirectional inverters that together with the ECC enable the described ac nanogrid to isolate the house from the utility grid (intentionally or due to a fault or other abnormal grid conditions), work in the standalone mode, and synchronize and reconnect house to the utility grid without load power interruptions. This also presents an opportunity for demand response operation in the grid connected mode, while in the islanded mode the inverters can perform frequency and voltage regulation of the ac line. However, major issues of the load sharing and stability in the above modes of operation still remain to be investigated. This work has been sponsored mostly by the CPES Industry Consortium. REN Mini Consortium Scope of Work All CPES Virginia Tech faculty members have been involved in the projects related to sustainable energy and advanced electronic power distribution systems, with individual efforts including: battery energy storage, wind energy, high voltage dc bus regulation, photovoltaic systems, 48 V dc bus issues, system control and energy management, hybrid ac/dc power systems, LED light packaging, dimming and wireless control, and design and control of bidirectional converters for grid interface and plug in hybrid electric vehicles. Much of the work has been sponsored by NSF and the CPES Industry Consortium, with additional support from ONR, Thales, Boeing, GE, DOE, VPT Energy Systems, Whirlpool, National Instruments, and Virginia Tech s Collage of Engineering and ICTAS. Companies interested in joining the REN mini consortium would be Principal Plus Members in the CPES Industry Consortium. These members pool resources and work jointly and synergistically with CPES researchers. Pre competitive technologies developed under this effort are shared among mini consortium members. The REN mini consortium would be established along the guidelines of the existing model of the Power Management Consortium (PMC): Annual Principal Plus Member contribution of $50K; Number of students involved would be proportional to the number of mini consortium members; Scope of work would fall within the topics outlined below, to be discussed and modified during regular progress reviews; Progress reviews are conducted three to four times per year, with at least one face toface meeting held in conjunction with CPES annual conference or other IEEE conferences, while others would be conducted via WebEx; As Principal Plus Members, mini consortium members have automatic membership in CPES Intellectual Property Protection Fund (IPPF). IPPF is a unique IP access mechanism 6

designed to provide all Principal level members with extraordinary IP advantage automatically, at no additional cost. Principal level members are invited to participate in quarterly IPPF telecons with CPES inventors to discuss invention disclosures and jointly decide which technologies to protect, with patenting costs covered by IPPF. Once a technology is protected, all Principal level members are granted a royalty free, non exclusive, non transferable license to use the technology disclosed during their membership years. The current ideas about the topics and the scope of work that could be covered by the research in the mini consortium are briefly described below. Future DC Nanogrid Although the contemporary house electrical architecture shown in Fig. 4 could provide significant improvements in energy consumption, being a part of the bigger electrical power system, it does cause a huge increase in the complexity of the power system that results from the coupled dynamics between thousands (even millions) of distributed actively controlled generation, storage, and consumption units. Today, there are practically no ideas on how resource agglomeration would happen in such systems or how to control them, especially when nanogrid penetration would approach 50%. A possible solution to this problem of complexity curse could be to put a bidirectional gridinterface converter in place of the energy management center (ECC) in Fig. 4. In that case, the whole nanogrid of the building is seen by the utility grid as a single electronic load/source, dynamically independent of the grid but dispatchable by the utility operator. The ECC is entrusted with the operation of the local renewable generation, load shedding, utilization of the static or mobile battery (PHEV) energy and other power management functions, as well as nanogrid stabilization and advanced, active islanding in the event of outages or other low frequency disturbances on the utility side. This approach could then be extended hierarchically so that a number of such semi autonomous nanogrids are combined to form a bigger microgrid system, which in turn is interfaced to a minigrid through a higher (substation) level ECC with high power bidirectional converter, and so on. In the proposed hierarchical grid architecture, the nanogrids are fully dynamically decoupled from the microgrid through ECC, so that their internal architecture is completely independent and can have different voltage, phase, and even frequency, from dc to kilohertz. Therefore, it could be envisioned that the future building electric system will be based on a dc nanogrid, as shown in Fig. 5. Compared to the conventional 50/60 Hz architecture in Fig. 4, dc nanogrid brings many advantages, starting with fewer power converters, higher overall system efficiency, and easier interface of renewable energy sources to a dc system. There are no frequency stability and reactive power issues, and no skin effect and ac losses. What is more, the consumer electronics, electronic ballasts, LED lighting, and variable speed motor drives can be more conveniently powered by dc. The future home dc nanogrid is envisioned to have two dc voltage levels: a high voltage (380 V) dc bus powering HVAC, kitchen loads, and other major home appliances, and a multitude of 7

low voltage (48 V) dc buses powering small tabletop appliances, computer and entertainment systems, and LED lighting. The 380 V dc level is chosen to match the industry standard intermediate dc voltage in consumer electronics with the PFC circuit at the input, so that conversion from ac to dc would involve only bypassing the front end rectifier. The 48 V dc level is chosen to coincide with the standard telecom voltage to facilitate adoption, increase efficiency, and provide enhanced safety when handling small appliances, while enabling aesthetically attractive designs with exposed electrical structural elements. Similar 380 V / 48 V dc power distribution systems are currently also being considered for datacom centers in Japan, Europe, and USA, and are also being contemplated for PHEVs and aircraft power systems. Several manufacturers already have on the market high power density bus control modules (BCMs) that supply 48 V from 380 V and are intended for these applications. Fig. 5: Conceptual dc nanogrid in a future home. In higher voltage dc systems, fault current interruption is of particular concern. However, in the proposed nanogrid architecture all power is fed from electronic power converters that are controllable and can provide active current limiting, thus reducing the need for electromechanical protection devices. The system could be even completely breakerless if all the source converter topologies comprise serial semiconductor switches which fail open in the case of abnormal failure. This would also eliminate the need for significant over sizing of the wiring and upstream converters that is traditionally used to ensure safe clearing of the electromechanical breakers in the case of faults. Therefore, such nanogrids may be able to provide increased energy efficiency, power density, and reliability, at possibly lower installation and operation costs. 8

CPES living lab is being designed with the described dc nanogrid in order to investigate issues and opportunities with future homes using dc systems. The new lab could be powered both by dc as well as ac with minimum rewiring, thus providing dual functionality and the ultimate in energy usage calculations and comparison for both systems. PV System A typical PV panel delivers 200 W maximum power, with less than 50 V output voltage. Most PV systems are configured with a number of panels connected in series/parallel to form multiple strings. The PV system is then connected to the grid with an interface inverter. Although this configuration is relatively simple, it has several drawbacks. First, the system cannot achieve peak power tracking (PPT) at the panel level. The PV system performance is compromised whenever a panel is shaded or ill functioning. The system lacks fault tolerance. To overcome these limitations, the micro inverter concept has been considered as a possible alternative. The micro inverter consists of two functional blocks: the converter block tracks the panel s peak power and the inverter provides the interface to the utility grids. However, existing solutions are rather complex with high cost. A more attractive alternative can be realized by separating the dc dc converter from the inverter function and integrating the dc dc converter with the PV panel to form a smart panel. Each smart PV panel can perform PPT, regulate the terminal voltage, protect the system against PV fault, and communicate and interact with other PV panels. To realize the idea of creating a smart PV panel, the dc dc converter must be made very small and cost effective, and suitable for integration with the PV panels. These smart panels can be configured in a variety of ways and connected to the grids through a simple, centralized converter, as shown in Fig. 6. (a) series (b) parallel (c) series-parallel Fig. 6: Solar modules with integrated converters locally controlled to optimize energy collection. This project offers an opportunity to utilize CPES s structural and three dimensional integration capabilities, as well as high temperature materials and processes, to cut cost, improve acceptance, and increase the overall energy harvesting efficiency of a renewable energy source for the future home. The size and weight issues are addressed not only by integration, but also by reduction of cooling requirements due to operation at elevated junction temperatures. 9

Solid State Lighting LEDs are expected to be the dominating light sources for use in residential and commercial applications, such as indoor/outdoor lighting, and backlighting for notebook, television, and all forms of displays. The LED driver generally contains three conversion stages (Fig. 7): the ac dc stage provides power factor correction; the dc dc stage provides regulation and isolation; while the third stage provides a current source to LED string with dimming capability. For high performance display, high dynamic contrast ratio is deemed essential for image quality. The dynamic contrast ratio can be improved in two aspects: the native LCD panel contrast ratio and the backlight driver response time. In order to achieve high dynamic contrast ratio, the linear ASIC solution is employed in essentially all current products. However, this method has high power loss, especially in the dimming operation. In order to replace the linear regulator with a switching converter, the latter must be operated with a switching frequency in the megahertz range. Fig. 6: (a) LED backlight driver with linear ASIC (b) Integrated LED module Wind Power Most wind power is transferred to the grid via variable speed doubly fed induction generators in large wind turbines, but the permanent magnet generator (PMG) interfaced to the grid through a full power ac ac converter with a dc link is being increasingly adopted due to its higher power density and better controllability. In normal grid connected operation, when wind power is relatively small compared to the grid short circuit power, the grid side converter is used to regulate the dc link voltage while the generator side converter regulates the PMG speed to achieve the desired power transfer, normally following a maximum power point tracking (MPPT) scheme. In the case of operating in a weak grid, or connected to a nanogrid, when wind power becomes a significant portion of the power system or even the sole energy source, the wind power generator and its converter are expected to help maintain the grid side voltage and frequency. In the case of the dc nanogrid, the generator side converter should try to adjust the generator output power to balance the load power need, so that the dc link voltage can be maintained. Given the variable speed and the nonlinear power characteristics of the wind turbine, the dclink control is a significant challenge under this condition. In the previous work, CPES has developed a novel control methodology for the dc link voltage by controlling the PMG speed 10

through the generator side converter. The energy relationship of wind turbine, PMG, dc link capacitor, and load was established. An intrinsic right half plane zero (RHZ), together with the wind power characteristics, the mechanical system inertia, and the dc link energy storage, were identified as the physical limitations for the control. The proposed control method in the weakgrid mode can perform well with changing wind speeds, can be combined with turbine pitch angle control, and can also transition seamlessly to and from the MPPT mode. However, operation in a dc nanogrid has not been investigated, especially in the presence of significant local fast energy storage, and in the presence of multiple other (weak) sources that also regulate and supply energy to the same dc bus. Plug in Hybrid Electric Vehicles Plug in hybrid electric vehicles (PHEVs) represent a new type of load and, when necessary, an energy source for future sustainable homes. During normal operating conditions, the home electric power distribution system must be designed to charge the vehicle batteries as economically as possible. However, the presence of the batteries and hybrid enginemotor/alternator system inside the vehicle also opens exciting opportunities for using PHEVs to provide convenient emergency power to the home in case of grid power outages. One of the important sustainable home topics for investigation is the compatibility of the power distribution system with plug in hybrid electric vehicles (PHEVs). CPES will investigate the converter hardware and power management issues associated with developing a robust electrical interface between the batteries on board an electric or hybrid electric vehicle and the electrical home power distribution system. One aspect of the project is to development of a high efficiency bidirectional power converter to provide the interface between the home electrical system and the battery pack on board the vehicle. In another aspect of the project, a study will be conducted to investigate the power management issues associated with this PHEV interface. The operating modes include use of the home electrical system to recharge the vehicle s batteries when needed and a discharge mode that converts the stored battery energy into usable power in the home following any unexpected loss of utility power. Energy Management The presence of the alternative/renewable energy sources and energy storage devices offers the opportunity to significantly reduce the net electrical energy consumption from the utility grid. Ultimately, the sustainable home will have the net zero energy consumption from the utility grid and zero emissions. For this to happen, the nanogrid system in the proposed future home must have the following characteristics: A mixture of renewable electric energy sources with the capacity to produce, at least, the net required energy utilized in the home, say, over one year interval, An energy storage system that can provide energy balancing on an average day between the intermittent renewable sources and varying loads, A bidirectional connection of the various alternative energy sources and storage devices to the grid that allows energy trading to provide economical incentive, 11

An ability to work in islanded mode and thus ride through most grid outages so that any desired electrical energy availability can be achieved. Controlling the system power flows and efficient energy management of various components in the future homes is critical. The study should include sizing of the renewable sources and energy storage devices, means of improving energy efficiency of various appliances and lightings and lighting controls, as well as the power distribution architectures of home based nanogrid. A desired scheme will require all hardware components to be smart and software components to be versatile with distributed intelligence and effective communication to the system. To achieve maximum benefits of energy utilization (from the cost and environment perspective) without compromising the living comfort, accurate dynamic and predictive modeling, at the behavioral level, of all major system components should be developed. Several known online and offline optimization approaches with centralized, hierarchical, or decentralized control need to be compared and experimentally evaluated. The existing CPES research on power management control strategies for various types of specialized electric power distribution systems and on agent based control methodologies in hybrid power systems will be modified and extended to the future home applications. Summary Based on the above mentioned scope of research, the planned tasks include but are not limited to the following aspects: PV System Smart PV Panel PV management system DC bus interface Micro inverters Plug in Hybrid Electric Vehicles / Battery Storage Bidirectional charger/discharger Battery management system DC bus interface Interface with ac grid Wind power Control in weak and islanded systems DC bus interface Interface with ac grid Energy Management for the nanogrid Energy management strategy Programmability and optimization of total energy utilization Wireless control and interfaces with various energy sources and appliances AC Nanogrid 12

- Cooperative control of grid interface converters with multiples sources and energy storage elements - Smoothless transition between grid connection and islanding - Hierarchical system modeling, analysis, and design DC Nanogrid High voltage (380 V) dc bus and low voltage (12 48 V) dc bus architectures Bidirectional dc ac grid interface converter DC fault current limiting DC outlets (prevention of arching) System protection DC DC converters (BCMs) Hierarchical system modeling, analysis, and design Solid State Lighting Smart LED with integrated electronics LED driver & power architecture for general lighting and street lighting Backlighting for large panel displays 13