Koichi Ito’s research while affiliated with Waseda University and other places

In Japan, a feed-in tariff scheme has been implemented to promote the use of photovoltaic power generators in combination with electric batteries. Although many researchers have assessed the operation of grid-connected photovoltaic systems with electric batteries, little attention has been paid to the impact of the electric batteries' degradation characteristics. This paper evaluates the operation of a grid-connected photovoltaic/electric-battery system in a house in terms of cost savings and energy savings. We constructed a long-term operational optimization model considering the degradation characteristics of the electric battery. This model contains two objective functions: energy savings and operating costs. We create a scenario of power rates and photovoltaic/battery system configurations. To reveal the optimal operational strategy for the photovoltaic/battery system, the multi-objective optimization problem was solved. As a result, Pareto-optimal solutions were obtained, and trade-off relationships between cost and energy savings are presented. In this study, the utilization of a grid-connected photovoltaic system with an electric battery was expected to be most effective in energy-saving priority operation.

The purpose of this study is to evaluate the maximum energy-saving potential of residential energy supply systems consisting of a solid oxide fuel cell (SOFC) cogeneration system (CGS) combined with a solar cell (SC) and a battery (BT), compared with a reference system (RS). This study applies an optimization theory into an operational planning problem to measure actual energy demands over the course of 1 year. Eight different types of energy supply system were compared with each other by changing the components of the SOFC-CGS, SC, BT, and RS. Meaningful numerical results are obtained, indicating the maximum potential energy savings.

The main objective of this study is to evaluate energy saving potentials of polymer electrolyte fuel cell cogeneration systems integrated with solar cell and battery installed into residential houses. There exist many alternative operational policies in the above systems, and the mathematical planning approach is taken to derive the rational system's operational solution based on the mixed-integer linear planning method. In the numerical analysis of this study, energy saving characteristics of 12 alternative systems are compared respectively, and as one of main obtained results, it become clear that the energy saving effects to install battery into the systems depend largely on the operational policy of the system together with the evaluation criterion of adverse electricity from the solar cell to the electric grid. In other words, in the case of giving high priority to the self-consumption of electricity from the solar cell within the house, the energy saving potential to install battery increases largely, but few effects are noticed in other cases.

In this study, a renewal planning problem of energy supply system is formulated as a large scale mixed-integer linear programming problem, in which the objective function to the minimized is the average value of annual total cost during system's evaluation period. By adopting the programming language AMPL and CPLEX solver, a numerical study is carried out for a hospital, where electrical(e.g., heat pump) and gas(e.g., gas engine cogeneration) systems are compared together with arbitrary combination one, which is composed of electrical and gas driven pieces of equipment, by focusing particularly on the influence of initial system's difference. The main results obtained are as follows: (a) If the initial system is gas one, it is better to renew it to the electrical one as soon as possible due to relatively low energy efficiency of gas utilizing pieces of equipment, the high price of gas input energy and so on. (b) If the initial system is electrical one, the optimal renewal year becomes relatively later year, because it is economically better to use the initially installed high efficiency system as long as possible. (c) Theoretically, the arbitrary combination system is of course the best renewal one. However, there is no economic difference between the arbitrary combination system and the electrical one.

In this paper, three conventional energy supply systems are compared from economic and energy saving characteristic points of view for an office building with total floor area of 5000m^2. These systems are consuming mainly electricity (E-system), heavy oil (D-system), and city gas (G-system) as primary energy source, respectively. For hourly and daily changing energy demands, the daily input energy cost or primary input energy is minimized through one year for these systems by considering disaster occurring three days, respectively. As a result, economic and energy saving superiorities of E-system are resulted to those of respective D- and G-ones. For disaster occurring situation, effects of installing solar cell and battery are also evaluated by changing capacities of them numerically.

Promoting utilization of photovoltaic (PV) cell, for residential use has become urgent issue recently. One approach is to use an electrical battery (BT) in the house in order to maximize consuming the electricity generated by the photovoltaic power generator. The energy systems including energy buffer like electrical batteries have a lot of operational flexibility. The mathematical optimization technique is useful for rational energy use The objective of this study is to making operational optimization models considering degradation characteristic of a battery. As a result, the four types of operational optimization models, which consider degradation characteristics caused by battery cycle life, were formulated on the mixed integer linear programming problems. Consequently, it was found that the appropriate optimal operational strategy should be derived from the model including the degradation characteristics on DOD.

When evaluating residential energy systems like co-generation systems, hot water and electricity demand profiles are critical. In this paper, the authors aim to extract basic time-series demand patterns from two kinds of measured demand (electricity and domestic hot water), and also aim to reveal effective demand patterns for primary energy saving. Time-series demand data are categorized with a hierarchical clustering method using a statistical pseudo-distance, which is represented by the generalized Kullback-Leibler divergence of two Gaussian mixture distributions. The classified demand patterns are built using hierarchical clustering and then a comparison is made between the optimal operation of a polymer electrolyte membrane fuel cell co-generation system and the operation of a reference system (a conventional combination of a condensing gas boiler and electricity purchased from the grid) using the appropriately built demand profiles. Our results show that basic demand patterns are extracted by the proposed method, and the heat-to-power ratio of demand, the amount of daily demand, and demand patterns affect the primary energy saving of the co-generation system.

The CO_2 emission characteristic is evaluated for 700W PEFC cogeneration systems used for residential houses based on the mathematical optimization method. First the mathematical model is built based on the measured data through one year. Then, for energy demand data measured at 10 houses through one year, the optimal potential of CO_2 emission reduction ratios are analyzed by optimizing the operational policy of the system. By comparing with the conventional system composed by the condensing gas boiler and electric grid, the reduction ratios of the CO_2 emission are ?5.42%〜-0.10%, and it is concluded that this system has no high potential to contribute for CO_2 reduction.

An electrically driven heat pump using CO^2 as the refrigerant is developed to supply hot water with a high coefficient of performance for use in buildings such as hospitals, hotels, and so on. In addition, these values of conventional heat pumps which use alternative chlorofluorocarbon gas are also increasing step by step in recent years. This study compares several alternative heat pump and hot water tank systems as well as a gas-fired boiler system from economic points of view based on a mathematical optimization method considering ambient temperature. A hospital is taken as the objective of this study, and the energy demand for hot water supply is estimated hourly and monthly through one year. As there are alternative modes for the system's operation, a mathematical optimization method is applied at this stage for responding to changing energy demands in a timely manner. The economic evaluation of the systems is done by applying the annualized costs method. Our results show that there exists an economically optimal combination of heat pumps and hot water tank of proper capacity. It is also seen that systems adopting heat pumps have advantages over gas systems from economic, energy saving and environmental points of view.

In this paper, a method of rationally determining an optimal renewal plan is proposed for energy supply systems based on the optimization approach. In the planning of this proposed method, it becomes necessary to consider many factors such as the deterioration of each pieces of existing equipment, candidate pieces of equipment with upgraded efficiency reflecting technology improvement in the proper renewal year, the structure of renewal system, and so on. It is also necessary to determine the system's operational policy to match energy demands, which change daily and hourly through the year. A numerical study is carried out for a hospital with the total floor area of 25 000m2, where electrical (e.g., heat pump) and gas (e.g., gas engine cogeneration) systems are compared together with theircombination of gas and electricity driven equipment (hybrid) system. Through this numerical study, the following results are obtained: a) Theoretically, the hybrid system is, of course, the best renewal system. However, the economic difference between the hybrid and the electrical systems is very small. b) The gas system is worse economically than the electrical one relatively due to the low energy efficiency of the related equipment, the high price of energy, and so on.