In order to achieve carbon neutral society, decarbonization of energy sector which emit over 80% of GHG in Japan should be the top priority. Energy policy must be based on quantitative analysis, and energy-economic models can mathematically analyze energy system. In this research, economically optimal energy supply-demand or flow and technology mix to achieve carbon neutral energy system in Japan is calculated by bottom-up technology selection model which is improved to consider carbon neutrality of energy system by considering CCU in detail. One-hour temporal resolution of energy supply-demand of household, commercial, and part of industry sector is also typical aspect of improvement of this research, which enable to analyze end-use in detail and endogenously calculate power demand especially important to carbon neutrality in one-hour temporal resolution. In this research, calculation scenario is set in terms of carbon fixation, the progress in development of geological storage of CO₂ and availability of carbon fixation to chemical products. Calculation results show that the capacity of carbon fixation influences fossil fuel utilization in system and determine the usage rate of carbon-free energy. Electrification of electrifiable services and decarbonization of oil-using services such as automobile and aviation is prioritized, and installation of synthetic methane is observed only the case which have less capacity of carbon fixation. Significant increase of power demand is predicted by electrification of end-use sector and typically by water electrolysis to produce hydrogen and DAC to capture CO₂.

View full abstract

The massive deployment of PV and wind, which provides decarbonized energy supply, bring about various additional challenges in operation of a power system. The congestion of transmission lines is an emerging issue which has been constraining new connections of renewable generation such as photovoltaic and wind. Recently, the rules to connect and operate generation have updated to accept new connection which occur congestion of transmission lines with the condition that the new-connected generation accept output control in case transmission congestions occur. Under this situation, production cost simulation including transmission constraints is a mandatory for planning and operation of a power system and for evaluation of each renewable energy project. In this study, the authors modeled demand and supply condition and transmission and simulated power flows of transmission lines in the Hokkaido in FY2019 to verify them against historical data.

View full abstract

The grid cost related to prediction errors in the power output of Variable Renewable Energies (VREs) is estimated to be 21 billion YEN in Japan. This issue needs to be addressed in order that VREs, especially for solar Photovoltaic (PVs) systems, become one of the major power resources in the Japanese electricity market. Energy management systems using demand side resources are expected to be one of the promising solutions, however, relatively little attention has been paid on the potential of residential fuel-cells (FCs) compared to the other sources including Electric Vehicles (EVs), heat-pump water heaters and batteries. This study highlights the potential of FCs for offsetting PV output imbalance caused by errors in day-ahead predictions, and the PV imbalance mitigation system was developed. In the system, the power output from FCs were controlled and adjusted remotely to offset the imbalance caused by PV through real-time monitoring. 1 MW PV systems and more than 3000 units of FCs are used to evaluate the developed systems. The results from this empirical research showed that the system could reduce the imbalance from 64% to 72% depending on the accuracy indicators used for the evaluation. In addition, we found the developed system can contribute to greenhouse gas emission reduction by both substitution of grid electricity and support of stable supply from PV as is pointed out in the previous study.

View full abstract

Road transportation has a good share in the global carbon dioxide emissions and models for estimating the vehicular emissions using physical parameters help to understand and potentially reduce them. However, such models at a regional level are usually insensitive to driving dynamics, as they are based on the average speed of the vehicles. The existing models considering the instantaneous speed and acceleration also use other factors like weather, vehicle parameters, etc., which involves many measurements. Furthermore, the estimation is done at large time periods of the order of several seconds. In this work, a real-time time series data is used to develop a model using only the vehicular speed and acceleration. It is based on a novel technique of using windows of the driving dynamics captured in a very short period, making some assumptions. The optimal drive features that influence the fuel consumption have been estimated using many machine learning regression models, validated, and compared. Among them, a multi-layer perceptron resulted the highest cross-validation of 0.64 using only the window of speeds, which is concluded to be reasonably good for practical estimation. Finally, these models are aimed to be applied in real applications based on J-credit and eco-driving.

View full abstract

In order to expand the use of renewable energy, a shift from "selling" to "self-consuming" of surplus electric energy generated by photovoltaic power generation (PV) is necessary. As for the means of the self-consumption, water heater with a hot water tank, which can store energy at relatively low cost as hot water, is attracting attention. Hybrid water heater, that has both an electric heat pump and a gas heat source, stores hot water to a relatively small tank compared to a simple electric heat pump water heater at less electric energy. Since it also has a gas heat source, there is no need to worry about running out of hot water even with a small tank. By combining a hybrid water heater and a storage battery system, an improvement of self power consumption ratio of PV energy is expected compared to the stand-alone usage of a hybrid water heater. This article reports the improvement result in an actual house. The self-consumption of PV energy used at the hybrid water heater was improved by 11.6 % in winter, 23.8 % in mid seasons, and 4.9 % in summer. Then the CO₂ emission is reduced by 33 % a year.

View full abstract

In this study, we analyzed the environmental impact of medium-rise apartments built using cross-laminated timber (CLT) and compared it to that of reinforced concrete (RC) housing. I found that CLT housing generates lower greenhouse gases (GHGs) than RC housing. To achieve further reductions, it is important to reduce the generation of GHGs during manufacturing of raw-material materials due to the use of biomass fuels in the drying process during the production of CLT. Furthermore, it is essential to reduce energy consumption during exploitation. A total environmental impact assessment was conducted using LIME2; the results showed that the total value of CLT housing had a similar impact as RC housing. To further decrease the environmental impact, it is essential to reduce the manufacturing load and extend the service life of materials used in new construction, specifically CLTs, joint fittings, and gypsum boards, and reduce energy consumption during exploitation. In addition, reducing the impact of building waste disposal is also essential, particularly in reducing the amount of waste generated for landfill disposal.

View full abstract

Public opinion is one of the most important factors in nuclear energy policy. However, it is difficult to understand concerns and needs that determine the public's attitude toward nuclear power plants (NPPs) through questionnaire survey. In this study, as the first step, the most important concern was extracted from the questions by the participants in the briefing sessions for residents by text mining. As the second step, changes in attitudes towards NPPs analyzed by questionnaire survey when the concern was resolved. The most important concern was "In case of an accident, it is possible to evacuate". Regarding restarting NPPs, proponents were more than opponents. Regarding constructing new NPPs, the number of opponents increased by 1.6 times and the number of proponents was reversed. Regarding constructing NPPs that do not require evacuation, which is a solution to the concern, the number of opponents decreased and the number of proponents increased, resulting in a balance of proponents and opponents. Respondents might feel uneasy about the novelty and unknownness of the question, which simply said, "Construct new NPPs." The government should set a clear purpose based on the public’s concern, such as promoting the development of NPPs that do not require evacuation.

View full abstract