The solar energy/utility interface

If dispersed solar energy systems are to become widely used, it is necessary that there be a harmonious interfacing with utilities. This paper summarizes discussions held during a series of workshops between 27 March and 11 May 1978. Representatives from utilities, solar manufacturers and public interest groups were convened to air their views in accordance with techniques used in conflict resolution. Although there was unanimity on only a few issues, there did result better understanding of the problems. The fundamental conclusion reached was that the federal government should provide technical assistance and guidance regarding dispersed solar energy but should otherwise not interfere with its choice and acquisition.     

Solar energy/utility interface: Technical issues

Typical solar heating, hot water, and air-conditioning systems cannot provide all of the thermal energy required by a structure. Auxiliary or “backup” energy is supplied by public utilities. Energy storage capability usually provided as an element in such systems may be employed for utility load management purposes. Technical issues relating to this are explored, including the need for development of standards to optimize the interaction between the solar system and the utility, the effect of variability in the utility's load profile upon the interface, and the impact of the use of solar storage for load management on solar systems. Passive solar heating systems are technically diverse. Technical requirements for interfacing passive systems with utilities in an optimum manner are discussed. Suggested approaches to improvement of this interface are reviewed. One solution to successfully interfacing passive systems with utilities is the development of hybrid systems. This potential is explored.     

An updated review of energy storage systems: Classification and applications in distributed generation power systems incorporating renewable energy resources

The demand of electric energy is increasing globally, and the fact remains that the major share of this energy is still being produced from the traditional generation technologies. However, the recent trends, for obvious reasons of environmental concerns, are indicating a paradigm shift towards distributed generation (DG) incorporating renewable energy resources (RERs). But there are associated challenges with high penetration of RERs as these resources are unpredictable and stochastic in nature, and as a result, it becomes difficult to provide immediate response to demand variations. This is where energy storage systems (ESSs) come to the rescue, and they not only can compensate the stochastic nature and sudden deficiencies of RERs but can also enhance the grid stability, reliability, and efficiency by providing services in power quality, bridging power, and energy management. This paper provides an extensive review of different ESSs, which have been in use and also the ones that are currently in developing stage, describing their working principles and giving a comparative analysis of important features and technical as well as economic characteristics. The wide range of storage technologies, with each ESS being different in terms of the scale of power, response time, energy/power density, discharge duration, and cost coupled with the complex characteristics matrices, makes it difficult to select a particular ESS for a specific application. The comparative analysis presented in this paper helps in this regard and provides a clear picture of the suitability of ESSs for different power system applications, categorized appropriately. The paper also brings out the associated challenges and suggests the future research directions.     

Citizen concerns with the solar energy/utility interface

Within the community of solar energy, utility rate reform, anti-nuclear, and other citizen groups there exists considerable disagreement as to what role, if any, is appropriate for the utility industry in solar commercialization. Some organizations have encouraged active involvement by municipal utilities and rural electric cooperatives while opposing participation by investor-owned utilities. Others have advocated an expansion of solar development programs spo/.nsored through the federal utilities such as the Tennessee Valley Authority. There are also sharp differences over whether utilities should be permitted to own, sell, lease, or install solar hardware. Similarly, there is much debate about whether utilities should finance solar installations or even provide information to their customers about solar technologies. Underlying these differing points of view are concerns about utilities monopolizing segments of the solar marketplace, discrediting solar technologies, goldplating the hardware, and instituting rate structures that discriminate against solar users. Some also feel that there is a basic philosophical conflict in having centralized institutions like utilities commercialize a decentralized technology such as active and passive solar heating equipment. Finally, a number of opponents have argued that there are preferable alternatives to utilities for promoting rapid solar energy use.     

Solar energy/utility interface: The technical issues

Technical issues will arise across the solar energy/utility interface as the use of solar energy penetrates further into our society. These are not the issues of simply collecting solar energy or of utility energy production but those issues related to the interconnection of solar energy systems with utility systems. The authors address this task by classifying solar technologies and then by setting a present utility baseline. Then they approach the matter of solar end-use technologies and solar generating technologies, with emphasis on technical issues of their interface. Finally, they discuss the challenges to the utility industry and describe a program of research and development that they consider necessary to meet these challenges.     

Towards 100% renewable energy systems: Uncapping power system flexibility

Relying almost entirely on energy from variable renewable resources such as wind and solar energy will require a transformation in the way power systems are planned and operated. This paper outlines the necessary steps in creating power systems with the flexibility needed to maintain stability and reliability while relying primarily on variable energy resources. These steps are provided in the form of a comprehensive overview of policies, technical changes, and institutional systems, organized in three development phases: an initial phase (penetration up to about 10%) characterized by relatively mild changes to conventional power system operations and structures; a dynamic middle phase (up to about 50% penetration) characterized by phasing out conventional generation and a concerted effort to wring flexibility from existing infrastructure; and the high penetration phase that inevitably addresses how power systems operate over longer periods of weeks or months when variable generation will be in either short supply, or in over-abundance. Although this transition is likely a decades-long and incremental process and depends on the specifics of each system, the needed policies, research, demonstration projects and institutional changes need to start now precisely because of the complexity of the transformation. The list of policy actions presented in this paper can serve as a guideline to policy makers on effectuating the transition and on tracking the preparedness of systems.     

Optimal interface based on power electronics in distributed generation systems for fuel cells

A hybrid system comprising a fuel cell stack and a battery bank was developed, built and tested in this research work. This hybrid system was built to supply both DC and AC outputs. The voltage levels set on electrical interconnection points are achieved with several power conditioning stages controlled by Pulse Width Modulation (PWM). The main advantage of this system is its excellence as a test bench, since it allows testing system performance at different voltage-restricted interconnecting points. Besides, power electronics are observed to play an essential role in distributed generation systems. The applications of the developed hybrid system extend from Auxiliary Power Units (APU) in vehicles (cars, buses or trains) to Uninterruptible Power Systems (UPS) in hospitals, nursing homes, hotels, office buildings or schools.     

Hydrogen-based systems for integration of renewable energy in power systems: Achievements and perspectives

This paper is a critical review of selected real-world energy storage systems based on hydrogen, ranging from lab-scale systems to full-scale systems in continuous operation. 15 projects are presented with a critical overview of their concept and performance. A review of research related to power electronics, control systems and energy management strategies has been added to integrate the findings with outlooks usually described in separate literature. Results show that while hydrogen energy storage systems are technically feasible, they still require large cost reductions to become commercially attractive. A challenge that affects the cost per unit of energy is the low energy efficiency of some of the system components in real-world operating conditions. Due to losses in the conversion and storage processes, hydrogen energy storage systems lose anywhere between 60 and 85% of the incoming electricity with current technology. However, there are currently very few alternatives for long-term storage of electricity in power systems so the interest in hydrogen for this application remains high from both industry and academia. Additionally, it is expected that the share of intermittent renewable energy in power systems will increase in the coming decades. This could lead to technology development and cost reductions within hydrogen technology if this technology is needed to store excess renewable energy. Results from the reviewed projects indicate that the best solution from a technical viewpoint consists in hybrid systems where hydrogen is combined with short-term energy storage technologies like batteries and supercapacitors. In these hybrid systems the advantages with each storage technology can be fully exploited to maximize efficiency if the system is specifically tailored to the given situation. The disadvantage is that this will obviously increase the complexity and total cost of the energy system. Therefore, control systems and energy management strategies are important factors to achieve optimal results, both in terms of efficiency and cost. By considering the reviewed projects and evaluating operation modes and control systems, new hybrid energy systems could be tailored to fit each situation and to reduce energy losses.     

Optimal configuration of power generating systems in isolated island with renewable energy

In isolated islands, usually diesel generators supply electric power. However, there are problems, e.g., a lack of fossil fuel, environmental pollution etc. So, isolated island, e.g. Miyako island, installs renewable energy power production plants. However, renewable energy power production plants are very costly. This paper presents an optimal configuration of power system in isolated island installing renewable energy power production plants. The generating system consists of diesel generators, wind turbine generators, PV system and batteries. Using the proposed method, operation cost can be reduced about 10% in comparison with diesel generators only from simulation results.     

Review on grid-forming converter control methods in high-proportion renewable energy power systems

Grid-forming (GFM) converters can provide inertia support for power grids through control technology,stabilize voltage and frequency,and improve system stability,unlike traditional grid-following (GFL) converters.Therefore,in future “double high” power systems,research on the control technology of GFM converters will become an urgent demand.In this paper,we first introduce the basic principle of GFM control and then present five currently used control strategies for GFM converters:droop control,...     

可再生能源发电的展望

21世纪能源结构将会发生根本的变化。寻找和开发利用清洁高效的可再生能源，将是世界能源发展的必然趋势。文章介绍了国内外新能源发电技术的现状与前景，针对中国所面临的挑战，提出了可再生能源发电问题的建议。     

Assessment of utility energy storage options for increased renewable energy penetration

Renewable energy technologies are expected to take the leading role in the forthcoming energy generation portfolio in order to achieve sustainable energy generation. The major constraints for increasing penetration of renewable energy sources is their availability and intermittency, which can be addressed through energy storage when available and energy use when needed. This work reviews the energy storage technologies and gives an up to date comparative summary of the performance parameters of the major energy storage options. The parameters compared here include efficiency, energy capacity, energy density, run time, capital investment costs, response time, lifetime in years and cycles, self discharge and maturity of each energy storage option. The choice of storage system will depend on individual requirements, and may even incorporate more than one energy storage system to increase the energy storage capacity and improve energy security.     

An energy management approach for renewable energy integration with power generation and water desalination

The share of the renewable energy sources (RES) in the global electricity market is substantially increasing as a result of the commitment of many countries to increase the contribution of the RES to their energy mix. However, the integration of RES in the electricity grid increases the complexity of the grid management due to the variability and the intermittent nature of these energy sources. Energy storage solutions such as batteries offer either short-term storage that is not sufficient or longer period storage that is significantly expensive. This paper introduces an energy management approach which can be applied in the case of power and desalinated water generation. The approach is based on mathematical optimization model which accounts for random variations in demands and energy supply. The approach allows using desalination plants as a deferrable load to mitigate for the variability of the renewable energy supply and water and/or electricity demands. A mathematical linear programming model is developed to show the applicability of this idea and its effectiveness in reducing the impact of the uncertainty in the environment. The model is solved for the real world case of Saudi Arabia. The optimal solution accounts for random variations in the renewable energy supply and water and/or electricity demands while minimizing the total costs for generating water and power.     

Life-cycle analysis of renewable energy systems

An imlementation of life-cycle analysis (LCA) for energy systems is presented and applied to two renewable energy systems (wind turbines and building-integrated photovoltaic modules) and compared with coal plants.     

Hybrid renewable energy systems for power generation in stand-alone applications: A review

It has become imperative for the power and energy engineers to look out for the renewable energy sources such as sun, wind, geothermal, ocean and biomass as sustainable, cost-effective and environment friendly alternatives for conventional energy sources. However, the non-availability of these renewable energy resources all the time throughout the year has led to research in the area of hybrid renewable energy systems. In the past few years, a lot of research has taken place in the design, optimization, operation and control of the renewable hybrid energy systems. It is indeed evident that this area is still emerging and vast in scope. The main aim of this paper is to review the research on the unit sizing, optimization, energy management and modeling of the hybrid renewable energy system components. Developments in research on modeling of hybrid energy resources (PV systems), backup energy systems (Fuel Cell, Battery, Ultra-capacitor, Diesel Generator), power conditioning units (MPPT converters, Buck/Boost converters, Battery chargers) and techniques for energy flow management have been discussed in detail. In this paper, an attempt has been made to present a comprehensive review of the research in this area in the past one decade.     

Novel method for setting up the relay protection of power systems containing renewable energy sources and hydrogen energy storage systems

Integration of renewable energy sources (RES) together with energy storage systems (ESS) changes processes in electric power systems (EPS) significantly. Specifically, rate of change and the lowest values of operating conditions during the emergencies are got influenced. Such changes can cause incorrect actions of relay protection (RP) as it was designed and adjusted with no regard for influence of RES and ESS. Detailed research on processes during the different normal and abnormal modes in both EPS and primary transducers and also in RP devices should be done to take preventive actions. To do this research mathematical modeling based on detailed and authentic models of all elements including RP should be used. HRTSim (which was developed by authors) software for simulating EPS provides the opportunity to create such models of EPS of any size without simplifications and limits. Using of this instrument together with detailed mathematical models of RP which were developed before provided the opportunity to investigate them rigorously in RES-integrated EPS. Settings providing adequate action of RP in certain conditions were performed as a result of this investigation. Fragments of these investigations are performed in this paper. Results of these investigations would be useful for designing new methods and tools of RP adjustment.     

Performance of building energy supply systems using renewable energy

There are two types of renewable energy widely used in China: air-source heat pump and compact all glass solar vacuum pipe water heating systems. To compare the performance of these supply systems, test systems were installed on two adjacent apartment buildings with the same structure, shape, and material. Both sets of equipment were placed in the same environment for the same 31-day period. The performances and performance parameters of the systems were systematically analyzed. The system energy consumption ratio of the compact all glass solar vacuum pipe water heating system was greater than that of the air-source system on 81% of the days in the study period. However, extension theory analysis showed that the weight coefficients for the performance parameters of the two systems were equivalent over the study period. The grey relational degree between the system performance and the parameters were also calculated. The grey correlation degrees of the compact all glass solar vacuum pipe water heating system's properties with outlet temperature, inlet temperature, environment temperature, solar radiation, and sunshine time were 0.69, 0.71, 0.68, 0.70, and 0.68; and the grey correlation degrees of the air source heat pump water heating system's properties with outlet temperature, inlet temperature, environment temperature, solar radiation, and hours of sunshine were 0.71, 0.73, 0.71, 0.65, and 0.72. Furthermore, multivariate regression equations were used to study the changes of other parameters when one of the single variables changes.     

Thermodynamic analysis of CAES/TES systems for renewable energy plants

A thermodynamic analysis is presented for an energy storage system without combustion and including thermal recuperation (adiabatic compressed air energy strong (CAES)). The storage volume is optimized and a system layout with thermal storage (TES) and variable configuration is designed. The proposed system is simulated in order to have a first estimate of its energy recovery efficiency.     

Reliability and economic evaluation of small autonomous power systems containing only renewable energy sources

Evaluation of reliability performance in every power system has to be done within a cost–benefit framework. This approach, however, is a very time consuming task, especially for systems that contain a large number of possible configurations, so simpler techniques referred to the calculation of reliability indices are used. In small autonomous power systems (SAPSs), such an evaluation uses mainly deterministic criteria. This approach, however, cannot be applied in SAPS that contain only renewable energy sources, due to the intermittent nature of the provided energy. In this paper, a complete reliability cost and worth analysis is implemented for these systems, combined with the calculation of some basic probabilistic indices, in order to discover their performance and propose the appropriate of them as a criterion of optimal system configuration. This paper proposes that normalized energy reliability indices as system minutes and energy index of unavailability can be used as adequate criteria of system's optimal performance. This conclusion is validated through a large number of sensitivity analysis studies that are based on different maximum annual loads and different mix of load types.