Akinobu Murata

Electrical energy storage systems for energy networks

Dispersed electrical energy storage systems are expected to work for load leveling, fluctuation smoothing, uninterruptible power supply and emergency power source. Their introduction seems to be essential in order to control the future complicated energy utility networks. In this paper, the characteristics of promising energy storage systems by pumped hydro, compressed air, secondary batteries, superconducting magnet, flywheel or capacitors have been surveyed and discussed. From the results, their characteristics and suitable application fields have been clarified.

Energy pay-back time and life-cycle CO2 emission of residential PV power system with silicon PV module

The concerns about environmental impacts of photovoltaic (PV) power systems are growing with the increasing expectation of PV technologies. In this paper, three kinds of silicon-based PV modules, namely single-crystalline silicon (c-Si), polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) PV modules, are evaluated from the viewpoint of their life-cycle. For the c-Si PV module it was assumed that off-grade silicon from semiconductor industries is used with existing production technologies. On the other hand, new technologies and the growth of production scale were presumed with respect to the poly-Si and a-Si PV modules. Our results show that c-Si PV modules have a shorter energy pay-back time than their expected lifetime and lower CO2 emission than the average CO2 emission calculated from the recent energy mix in Japan, even with present technologies. Furthermore the poly-Si and the a-Si PV modules with the near-future technologies give much reduction in energy pay-back times and CO2 emissions compared with the present c-Si PV modules. The reduction of glass use and the frameless design of the PV module may be effective means to decrease them more, although the lifetime of the PV module must be taken into account.

An evaluation on the life cycle of photovoltaic energy system considering production energy of off-grade silicon

In this study, single-crystalline silicon (c-Si) photovoltaic (PV) cells and residential PV systems using off-grade silicon supplied from semiconductor industries were evaluated from a life cycle point of view. Energy payback time (EPT) of the residential PV system with the c-Si PV cells made of the off-grade silicon was estimated at 15.5 years and indirect CO2 emission per unit electrical output was calculated at 91 g-C/kWh even in the worst case. These figures were more than those of the polycrystalline-Si and the amorphous-Si PV cells to be used in the near future, but the EPT was shorter than its lifetime and the indirect CO2 emissions were less than the recent average CO2 emissions per kWh from the utilities in Japan. The recycling of the c-Si PV cells should be discussed for the reason of the effective use of energy and silicon material.

Operational strategies of networked fuel cells in residential homes

Residential fuel cells (FCs) have been recently launched in the commercial markets in Japan. The introduction of FCs in residential homes has become a realistic option. We previously proposed energy networks that enabled the interconnection of homes via electricity, heat, and hydrogen networks in order to allow consumers to interchange energy. Further, we have developed a PC-based simulator and an experimental system to evaluate the proposed energy networks. In this paper, analyses on the strategies of the electricity dispatch of FCs and their load sharing in the interconnected homes equipped with the energy networks were performed. The analyses were conducted with the experimental system, PC-based simulator, and a newly constructed mathematical model that optimizes the operation of the FCs with the minimization of both annual energy cost and CO2 emission. The basic characteristics of FC stacks were obtained by experimentations, and these were stored in the PC-based simulator. Four different operational strategies were carried out for four FC stacks, and these strategies were compared. It was concluded from the results that the operational strategies, which have different advantages with regard to efficiency in electricity generation and heat recovery, should be applied depending on the seasonal demand variations

Impact of forecast error of photovoltaic power output on demand and supply operation in power systems

Increasing the proportion of power generation from renewable energy sources has become increasingly important in Japan since the nuclear accident caused by the 2011 Tohoku earthquake. Photovoltaic (PV) generation in particular has gained much attention in Japan. In general, the supply and demand of electricity are maintained by economic-load dispatching control (EDC), which refers to regulation of the output of conventional (thermal or hydro) power plants to minimize their operational costs. Using the PV generation forecast in EDC, which includes the unit commitment (UC) of conventional power plants, is essential to maintain the economy and reliability of the power system with large-scale integration of PV generation. In this study, we consider an EDC that determines the UC on the basis of the day-ahead PV generation forecast. The frequency and trend of outages and power surpluses due to the forecast errors of the PV power output are evaluated.

Application of battery energy storage system to power system operation for reduction in pv curtailment based on few-hours-ahead PV forecast

The output suppression of PV generation (PV curtailment) is important to maintain the supply-demand balance in power systems. However, the energy loss due to PV curtailment will become larger in the future. In this study, we propose and evaluate an operation scheduling method for a battery energy storage system (BESS) on the basis of the day-ahead and intraday few-hours-ahead PV generation forecasts for a reduction in the PV curtailment. The BESS is used in the economic-load dispatching control (EDC) in power systems, which considers the unit commitment (UC) of conventional power units. Numerical simulations are carried out to evaluate the proposed method. The simulation results show that the BESS operation based on the intraday few-hours-ahead forecast method is effective for the efficient use of PV energy.

Evaluation of Relationship between Demand Side Management and Future Generation Mix by Energy System Analysis

Demand side management is an important part of the smart grid for alleviating the uncertainty in renewable energy generation including such as wind power generation and photovoltaic generation. The nuclear accident caused by the 2011 Tohoku Earthquake forced a re-evaluation of the future power generation system in Japan. As a result, demand side management has become critical for future Japanese power systems with its large integration of renewable energy sources, and the evaluation of the impacts of this management is needed for planning purposes. This paper proposes a new energy system model that expands the function of MARKAL (Market Allocation) and considers the characteristics of the total power system in more detail. The relationship between demand side management and the future power generation mix was evaluated using energy system analysis as part of the proposed model. The electric water heaters and heat pump water heaters are conceived as the controllable loads in the demand side management.

Power system operation with battery charge/discharge scheduling based on interval analysis

AbstractThe use of photovoltaic (PV) generation forecasts in economic load dispatching control, which includes the unit commitment of conventional power plants, is essential to ensure the economic performance and the reliability of power systems. In the previous study, we developed a day-ahead charge and discharge scheduling method of battery energy storage systems based on interval analysis using prediction intervals of a PV generation forecast; this interval forecast considers forecast errors and gives not only the forecasted output but also the possible range of the actual output with a certain confidence. In this study, we evaluate the proposed scheduling method by numerical simulations in terms of the power system supply and demand operation.

Evaluation of economic-load dispatching control based on forecasted photovoltaic power output

In Japan, the issues of whether to phase out nuclear power plants and install generation from renewable energy sources on a large scale is being debated owing to the nuclear accident caused by the 2011 Tohoku earthquake. Attention is particularly focused on photovoltaic (PV) generation-a technology for which Japan is renowned. However, large-scale integration of PV generation may cause an imbalance between the supply and demand of electricity in power systems. Using the forecasted PV power output in economic-load dispatching control (EDC) is essential to maintain the economy and reliability of power system operation. The focus of this paper is placed on an EDC that determines the unit commitment (UC) on the basis of the day-ahead PV generation forecast by numerical simulations. Suppression of the PV power output and installation of a battery energy storage system (BESS) are also considered.

A network of residential fuel cells and operational strategies: evaluation with a PC simulator and an experimental system

Introduction of fuel cells into residential homes would be a realistic option of near future in present state in Japan. Research and development on polymer electrolyte fuel cell (PEFC) for residential use is at the final stage toward commercial market launch. This paper presents the concept of the interconnection of residential homes with energy networks, and a PC simulator and an experimental system for evaluation of the advantages and disadvantages of the networks, and the result of simulation and experimentation with those evaluation tools. The homes can share their energy equipment virtually by the networks. It provides flexible and efficient operation of the equipment, and reduces partial load operation and start-stop operation which damage the efficiency or the life time of fuel processors. Some operational strategies of the fuel cells were proposed and evaluated with both simulation and experimentation. The evaluation result also revealed that fuel cell systems should be installed to not all homes but some homes within residential areas to realize efficient operation.