This paper presents the results of the survey to initially assess the current situation of power supply chain development in Vietnam. The research team used questionnaires to identify challenges and barriers in power production
investment. Surveyors came from power generation units, power transmission and distribution units, service providers for power plants, consulting agency and research institutes. The results indicated that there is a difference among enterprises' barriers and difficulties in the management, finance and investment environment. This difference is related to the business profiles of the enterprise, its segment of the electricity generation market, capacity and experience in the market.
The utilization of hydrogen as an energy carrier is promoted in major countries as one of the effective climate change
countermeasures. For the effective use of hydrogen, emissions reduction effect must be evaluated throughout the entire supply chain. The calculation method of greenhouse gas (GHG) emissions and reduction effect have been standardized by the Ministry of the Environment, Japan (MOEJ). MOEJ has developed the LCA (Life Cycle Assessment) Guideline for hydrogen producers, distributors, and end users to evaluate their businesses in Japan. Additionally, the LCA Calculation Tool, manual, and samples were provided and made available on the website of MOEJ.
This paper is investigating the CO2 emission saving of ground source heat pump (GSHP) systems for providing heating comfort condition for the residential buildings in cold climate of Japan. After presenting the data related to space heating energy consumption in Japan and also the concept of GSHP system, the required energy and equivalent CO2 emission of heating system for a sample building in Sapporo City were calculated in 3 scenarios: GSHP system, air-source heat pump (ASHP) system and kerosene heaters. Results of the calculations showed that the energy consumption and CO2 emission in the GSHP system is lower than the other scenarios. The CO2 emission in the kerosene scenario was 45% and 36% higher than GSHP and ASHP scenarios, respectively.
Feed-in tariffs (FITs) have been considered as the best support mechanism to rapidly increase renewables production in the EU. Against this background, the paper looks at the roles of FITs on accelerating renewables industry and mitigating climate change in the EU and analyses the advantages and barriers in Germany and the UK. On the other hand, based on the status of FITs performance in China, the study will also discuss what references from the EU could provide for China in further promoting renewables to make more contribution on mitigating global climate change in future.
Japanese energy system towards mid-century has been assessed using optimization type energy model TIMES-Japan. The assessment framework couples an energy system model and detail sectoral models such as power system model, wind and solar power capacity potential models, and building energy model, via soft-linkage. Aggressive introduction of wind and power under long-term CO2 constraint would accelerate low-carbon electricity supply and end-sector decarbonization.
This study shows two scenarios with rich renewable energy supply in 2050 for Japan. ‘Bridge scenario’ shows 80% decrease of greenhouse gas emissions from 1990 level. ‘100% renewable energy scenario’ shows energy supply fully with renewables to all of the end-use energy demand. The future energy demand is estimated by activity level with decreasing population, change of industrial structure and efficiency improvement. The results of energy demand for two scenarios in 2050 show 53-61% of 2010 level. We surveyed the potentials and prospects of domestic renewable energy sources such as hydropower, photovoltaic (PV), wind, geothermal, biomass and solar heat. Based on the survey, we synthesized two scenarios by allocating renewable resources to energy end use. We performed dynamic hourly simulations of renewable electricity supply to the electricity demand using weather data. The excess electricity caused by variable generation is used to supply energy to EV, FCV and thermal energy demand.
In this paper, we conducted quantitative analysis of the power supply system in Japan on the premise of massive introduction of variable renewable energy (VRE) for 2050. In the analysis, electricity supply simulation model, of which objective function is minimization of social cost, that can consider ensuring demand and supply balance and control reserve and restriction of inter-connection line capacity was used. As a result, in order to make effective use of the amount of electricity generated by renewable energy, it is important to adjust its placement to demand. Hydrogen production plant and methanation plant should be considered under mass VRE introduction.
Japan's electricity business now faces some trends and challenges that lead to inevitable and irreversible transformation. These are Deregulation, Decentralization, Decarbonization, Digitalization, and Depopulation, which can be called 5Ds. TEPCO calls the future image of the electricity business led through these 5Ds Utility 3.0, and in 2017 published its concept in the form of a book. The trigger of change is Decentralization, that is, exponential price destruction of DERs (Distributed Energy Resource) that do not emit CO2. These can bring the most energy abundant era in human history, but in order for the system to be sustainable, Digitalization and Re-powered Deregulation are necessary. Digitalization can also provide a solution to the need for convergence and productivity improvement of infrastructure services arising from Depopulation.
Renewable deployment in Japan has been rapidly increasing owing to the introduction of the Feed-in-Tariff Program in 2012. This paper describes the situation of renewable energy deployment in Japan as well as challenges and potential solutions toward massive integration of renewable energy.
This paper describes that electric power systems in the world are transforming to “decentralized” and “democratized” systems and that, under such systems, aggregation of various distributed energy resources including renewable energy sources and a grid code, which is a set of rules and requirements for some characteristics applied to all generatorsand resources, are crucially important. The precedent examples of the grid code issues and current situation in Japan are reviewed to remind the stakeholders of the importance of aforementioned two elements.
Fish cultivation experiment, and sea cucumber cultivation system that adapted renewable energy was designed in Aomori prefecture, located in the Northernmost part of Honshu, with an aim of creating a new fishery in fishing ports without any power supply systems. Fish cultivation experiment was conducted on October 2017, at the Kita-Kanegasawa fishing port. The devices that were installed near a fishery cage captured 1,595 horse mackerels. Water temperature sensors, and communication devices were installed for monitoring the water quality. In addition, batteries, an air supply pump, a wind powered generator and solar panels were installed for maintaining a cultivating environment. A sea cucumber cultivation system was designed, because sea cucumbers have an aversion to air, to confine in the water area of the fishing port by transporting air from an air supply pump to the concrete blocks creating air layers, which is installed at the mouth of fishing port. “Rotation flow Type” turbines and blocks are currently in production. This system was installed at Ishizaki fishing port in March 2018.
Small hydropower plant has been winning attention in Japan after 3.11 (Great East Japan Earthquake). This paper is aimed at analyzing impact of small hydropower plant on local economy using a project in Tsuchiyu Hot Spring in Fukushima Prefecture as a case. Tsuchiyu Hot Spring sustained damage caused
by harmful rumors or misinformation after 3.11. To revitalize the deserted hot spring town, the local community
decided to construct one small hydro power plant and a binary power plant. This paper analyzed what kind of
impact these two renewable energy power plants brought to the hot spring site.
After proposing “Biomass Asia Strategy” and arranging “Biomass Asia Workshop”, we have promoted bilateral collaboration of biomass energy between ASEAN countries and Japan under support of NEDO, JICA and JST. We also discussed region wide collaboration on biomass energy at “International Policy Dialogue”. After Paris Agreement, our “Biomass-Asia Project Team” in the Engineering Academy of Japan made the policy proposal to ASEAN countries. Now we are promoting our e-ASIA project among ASEAN countries and Japan (Feasibility Study on Social Implementation of Bioenergy in East Asia) as international industry-academia-government collaboration.
Vietnam is relatively rich in renewable energy resources. Renewable energy (RE) is considered to play a significant role in providing electricity services to rural communities in Vietnam where many households are still without access to the National Grid. Those suitable for electricity generation include small hydro, solar, biomass, wind and geothermal. Nowadays, the Government of Vietnam encourages clean energy development in an effort to fulfill obligations and commitments under the UNFCCC and Paris COP21 agreement, continuing to implement the National strategy, programs and plan in response to climate change. At the moment, RE practices are implemented mainly driven by donor funds and grants whilst commercial investment and local community initiatives is still limited. The paper reviews constraints on RE practices in rural community through small hydropower and biogas case studies in Hoa Binh, Quang Nam and Lam Dong provinces of Vietnam. It then proposes key recommendations for RE development at community level in rural Vietnam.
Given the prevailing issues of poverty and inequality in rural areas of developing countries, as well as their vulnerability to the effects of climate change, we focus on discussions regarding rural electrification policy in a case study of rural Kenya to ensure long-term social well-being. Four electrification projects are assessed using a sustainability framework in order to evaluate and compare their performance in the context of the promotion of socio-economic development. The assessment reveals that the private sector projects are the better performers, a result which encourages the relevant institutions to provide further support for their continued growth and development.
This study estimates the net cost effectiveness of feed-in tariff (FIT) for CO2 reductions by diffusion of solar PV in Japan’s residential sector during the period from 2009q1 to 2014q1, using 47 prefectural panel data for properly excluding the contribution of capital subsidy, another large-scale diffusion policy. The results show that the net cost effectiveness of FIT was lower than the gross one by a factor of 2-3, and that FIT was less cost effective than capital subsidy primarily due to FIT’s policy costs accumulating over time, although the impact of FIT was larger than that of capital subsidy.
Rapid population and economic growth in Kenya have resulted in rapid rise in energy demand. The main challenge facing the government is how to meet increasing demand in spite of environmental and budgetary constraints. This paper argues that demand side management and smart grid are the most suitable approaches to the future power demands in Kenya. It raises a question as to whether Kenya’s current energy supply capacity can meet current power demand without installing additional power plants. It concludes that with demand side and smart grid management practices, Kenya may meet its current demand from current installed capacity.
The concept of global governance gained importance after it was recognized that individual states were ineffective at finding solutions to transnational problems because of growing global interdependence and rapid technological developments. Environmental degradation and climate change are among the transnational issues that necessitate an effective global governance system. International cooperation and increasing the share of renewable energy in global energy consumption by the help of an effective global governance system is essential to prevent environmental degradation and climate change. International initiatives and organizations make valuable contributions to advance the share of renewable energy sources in global energy consumption. Nonetheless; legitimacy, efficiency and effectiveness of these organizations and initiatives should be enhanced. In this connection, role of the G-20 is critical. The G-20 may become a platform to enhance legitimacy, efficiency and effectiveness of the global governance system. Important steps such as initiating the G-20 summit process were taken but further steps such as increasing voices of emerging countries in the governance structures of key international organizations should be taken.
This study investigates an optimal deployment of variable renewables (VRs) in massive decarbonization in Japan’s energy system toward 2050 under several carbon-dioxide (CO2) mitigation scenarios. A multi-region bottom-up technology mix model which incorporates a high-temporal-resolution power sector is developed and utilized. The model is formulated as a large-scale linear programming model with 14 million endogenous variables and 30 million constraints. Simulation results reveal that severer requirement for CO2 reduction promotes decarbonization in the power sector and electrification at final demand sectors as well as energy conservation. The deployment of VRs increases as the carbon constraint gets rigorous and the large amount of VRs accompanies huge installation of batteries while suppressed electricity also increases due to intra-regional imbalance in demand and supply, and capacity limitations of inter-regional transmission lines.
Renewable electricity is expected to be crucial for mitigating climate change; yet, despite its abundant potential globally, renewable electricity faces system integration challenges due to spatially-imbalanced resource distribution and the intermittent nature of its output. As technical countermeasures, energy carriers, including hydrogen (H2) and ammonia (NH3), gain attention to control the variability and transport renewable-based energy for a long-distance. This study aims to assess the economic viability of H2 and NH3 for integrating renewable electricity in Asia—the gravity center of global energy market—employing an energy system model with a detailed spatial resolution. Simulation results suggest that cost reduction for electrolyzer would be prerequisite for promoting H2 production using renewable electricity (RE-based H2); limited availability of other low-carbon measures, including biomass and carbon capture and storage, would be a driver as well. Water electrolysis contributes to absorbing seasonal variability of renewables, and the produced H2 is mainly for decarbonizing the gaseous final energy demand. Pipeline transportation dominates H2 trade due to high fixed costs for maritime transportation. The simulation results also imply economic challenges for NH3 as energy carrier, again due to additional costs and energy losses for long-distance transportation.
For long-term sustainable de-carbonization, not only the improvement of conventional technologies such as thermal power generators but also customer side technologies such as renewable energies, storage batteries, demand response, etc. will play an important role. In order to verify the value of increased deployments of renewable energy resources and promote the dissemination, integrated impact analysis of the resources on electric power systems, as it is called Integration Studies, are becoming important. The authors investigated domestic-and foreign-examples of Integration Studies, and show the result of an Integration Study for Japanese power systems. This paper discusses the way to realize higher variable renewable energy deployment and lower carbonization society in the near future.
Japan started full-scale renewable energy after two oil shocks. The government take the efforts into the Sunshine plan (new energy) and Moonlight plan (energy conservation). We promoted research and development of industry, academia and government collaboration. However, MITI just focused on sunlight only in renewable energy in 1993. Since 2005, it was a dark ages for renewable energy, partly excluding biomass. The introduction of renewable energy is now progressing with the introduction of the FIT system in 2012.
This survey was performed to study the effects obtained from renewable energy power generating equipment that has been installed according to the Government-Funded Project Promoting the Use of Renewable Energy subsidized by the Ministry of the Environment (MOE). The evaluation assessed the following items: (1) operational status; (2) management
status; (3) CO2 reduction; (4) cost of CO2 reduction; and (5) ripple effects. It was found that equipment that had been installed in association with the establishment of new buildings and that which had been included in the energy and emergency-response plans of the regions both tended to have high scores.
This paper proposes that even under circumstances in the future where renewable energy plays a major role as energy sources, the excellent characteristics of natural gas, gas-related infrastructures and gas-utilizing equipment will be able to harmonize with those renewable energy sources and have huge potential of contributing to achieving the realization of zero-emission society.
The Chinese government promotes biogas systems in order to combine biogas technology with agricultural production and environmental protection. However, the biogas slurry still cause significant pollution. Because individual large-scale livestock farmers seem to lack sufficient recognition and strength to handle all concerns including biogas and pollution. So the aim of this paper is to present the status of biogas production related to multi-actor biogas utilization, and evaluate the biogas system of China, compared to those of Japan.Then we have suggested the future perspective to government building and operating a comprehensive utilization system, which is supported by a multi-actor partnership.
For the rural energy systems, bio-mass energy from animal husbandry is one of the key resources to developed in the near future. For livestock raiser, especially poultry raiser, currently improvements in raising barn and surrounding environments are inevitable, as regards foul odors and water pollution due to livestock excreta. We developed revolutionary product “GT-S liquid “for solving those issues and here we report on our current results on growth rate, feed intake, feed conversion, production score, meat yield, and feed cost of poultry, together with the quite high quality and excellent properties of GT-S liquid as a totally natural and safe product.
This study shows the state of the energy transition towards 100% renewable energy in Japan. The status and support policy of renewable energy are described with issues to reform existing electrical power system for energy transition. As social aspects, issues facing electrical power system reform and social acceptance are discussed. Policy feasibility of an energy transition is shown as development of FIT scheme for renewable energy. Furthermore, long-term vision and community power for promoting an energy transition are discussed with strategy and indicators for 100% renewable energy region. As conclusions, policy recommendations are proposed for promoting energy transition towards 100% renewable energy in Japan.
We investigated the annual degradation rates of six types of crystalline silicon (c-Si) photovoltaic (PV) modules under subtropical coastal climate conditions based on two indices: outdoor performance ratio under high and stable solar irradiance conditions and indoor power output measured under the standard test conditions (STC). The results indicate that the PV modules composed of p-type c-Si solar cells with aluminum back surface field (Al-BSF) tend to show the lower annual degradation rates of 0.3%/year or less, although the PV modules consisting of n-type c-Si solar cells tend to show the higher degradation rates from 0.6 to 1.2%/year.
Measurements of failed PV modules under different operating temperature delineated the fact of output power reduction which becomes obvious only at high module temperature. Interconnect disconnection caused by cracks with module temperature dependence is one of the reason of the power reduction. Output power reduced modules under high operating temperature with disconnection of interconnect may pass the module performance warranty criteria at STC measurements. To evaluate the actual module performance correctly and fairly, not only the STC measurements for warranty criteria but also establishing the certified methods for on-site diagnosis of performance and the regulations for on-site inspections are required.
Photovoltaic (PV) module surfaces are suffered from dust deposition and reflection that can cause reduction in efficiency of PV. Therefore, the impact of reflection and soiling must be taken into account in PV performance. This study describes reliable surface coating with superhydrophilic properties that has no degradation on PV systems. We used nano-porous silica films with antireflective and anti-soiling effects on the surface of CIGS PV modules. The coating treatment was performed with single-layer coating on installed modules on Dec, 25 2014 and exposed outdoors to analyze the coating performance. The output of the array increased after coating than before coating. Besides, over 3 year‟s performance of antireflective coating was evaluated and we observed no degradation. Therefore, antireflective coating showed its long term effect of reliability on PV module surface.
We investigate degradation related to interconnector for crystalline Si PV modules after DML test or PCT. The module including the thin glass of 0.85 mm-thickness shows the dark EL area spreading along the bus bar electrodes of cells after the DML test, which is due to separation of a soldering between interconnector and module bus bar ribbon or a fracture of interconnector near ribbon wire.The crystalline Si PV modules after 600 hours in PCT shows generation of dark EL area from bus bar electrode of PV cells, and drop in FF, depending on the amount and type of flux when interconnector is tabbed on the bus bar of cells. Cross-sectional EDX analysis detect the presence of Sn and O on the surface of the Ag finger electrode in the dark EL area.
The introduction of photovoltaic systems is progressing rapidly with the expansion of demand for renewable energy. However, in recent years accidents are seen in some systems due to natural disasters caused by strong winds and large typhoons. On the other hand, wind resistance tests have not been established to evaluate the validity of the support structures for PV system. In this paper, we report the result of evaluating the structural strength of a real scale ground installing type solar cell array by wind resistance test using dynamic wind pressure equipment which is the world's largest scale.
Automated diagnostic systems are thought to be promising for optimizing the operation and reducing the maintenance cost of mega-watt solar power plants. String powers and meteorological parameters such as irradiance can be measured by the systems without requiring relatively high cost. We have measured string powers and irradiances at short intervals in an operating mega solar power plant. We have investigated the measurement data accumulated for about two years and found a fairly large number of periods of time showing rapid variations of the irradiance and the string power.
Module Level Power Electronics (MLPE) is dominating the residential photovoltaic market in the US. Safety functions to meet the local regulation is believed to be a major driver of this success. MLPE, however, has other advantages against traditional string inverters such as mismatch loss mitigation, flexible string configurations and module level monitoring. These features are also important and provides for many beneficial advantages for commercial applications. Especially in Japan, the DC optimized Inverter solution is used mainly for ground mount commercial applications including megawatt scale power plants. These projects clearly demonstrate MLPE’s potentials for megawatt scale PV market.
In large-scale PV systems, remote monitoring is conducted in order to reduce the cost of on-site inspection. We propose a remote strings fault detection method for PV systems. By measuring the string current and module voltage of each strings, the proposed method can detect fault such as partial disconnection in the string. The current / voltage measured at the power conditioner and the threshold value obtained by simulation are used for detection. We verified the accuracy of proposed method using an actual large scale PV system in Japan and detected a string that may contain a malfunctioning module.
IEC TC82 WG7 works for standardization of concentrator modules, and their optics, mechanics and other advanced photovoltaic systems. These standards will be in the general areas of safety, photoelectric performance and environmental reliability tests. The standards ultimately produced should be universal and non-restrictive in their application, taking into account different environments and manufacturing technologies. In addition to the essential electrical and mechanical characteristics, standards will be written for other relevant factors such as thermal performance, high voltage performance, angular performance, mechanical precision, specific testing fault resistance and fault-tolerant design. Recently an expanded group started to discuss standardization of the car-roof PV.
Electricity industries worldwide are undertaking the challenging transition towards more sustainable energy demand and supply, including further electrification, optimum generation mix changes and the development of innovative system structure and technologies. In order to accommodate new demand and the variable renewable sources such as PV and wind, there are emerging needs to enhance the system architecture and system operation. This paper, as apart of Activity 3.5 of Task 14 “High-Penetration of PV Systems in Electricity Grids” of International Energy Agency (IEA), Photovoltaic Power System (PVPS) Program, surveys the emerging requirements in integration studies of innovative transmission system operation of high penetration of PV.
With the advancement of both vehicle and PV technologies, it will be possible that 70 % of the cars will run by the solar energy, 8 % of the Greenhouse gas emission will be cut, and 50 GW/year of the new PV market will be created. The R&D of the car-roof PV impacts to entire PV technologies. The progress of R&D of the car-roof PV impacts to various aspects of PV technologies including 3-D nowcasting leading to the establishment of the real-time market and smoothing PV output to the grid.
Kyushu region in Japan is an area of interest in power systems analyses, with low power demand, and high levels of photovoltaic power penetration. We present the interannual variability of day-ahead forecasts of solar irradiation for Kyushu, using four year of data. Forecasts were done with a method based on machine learning. The results show that the maximum variation of the root mean square error reached 10.5%. Monthly, the mean forecast errors of February, May and September had little interannual variability. The results indicate that there are months in which the accuracy can be estimated with good confidence.
In order to provide stable power system operation, our work is focusing on the development of a model to estimate Global Horizontal Irradiance (GHI) to create a day-ahead prediction for photovoltaic power output, as well as the prediction of the forecast error itself. The knowledge about the relation between single and combined parameters of weather forecast data to solar irradiance plays a key role in our work and is being analyzed. Our results found contributes to creating a reliable forecast model and improve energy system stability. As a future work, a pattern-analysis of outliers has to be conducted for further improvements.
It is important to forecast photovoltaic (PV) power output for the stable and efficient grid operation, intelligent household energy management, and so on. In this paper, a new forecasting method using real PV outputs power data of multipoint PV systems is proposed. In this method, first, the output power data of respective PV systems are normalized. Then, the future motion of the geographic distribution of the normalized values is estimated. Finally, future normalized values are re-converted to future output of respective PV power systems. We assess this method by analyzing errors between actual PV output and predicted PV output.
This paper presents a study on the influence of Sun-Earth geometry and atmospheric variables on the predictions of solar global irradiation (GHI) obtained from the ECMWF model. It was found that the differences between predictions and measurements of GHI are correlated mainly with the clearness index, solar zenith angle, mean air temperature, relative humidity and total water column. An artificial neural network is developed to improve predictions of GHI for four locations being the base for a predicting algorithm that can be used in energy management models of solar systems thus allowing a better management of renewable energy conversion.
In the future, power to gas process chain could play a vital role in the energy system because of long-term storage of solar energy and an effective transformation to usable gases. The resulted energy from the photovoltaic system can be transformed into useful gases such as hydrogen and methane which can be used in gas grids, auto-motives and industries. To use hydrogen as a useful, artificial, sunlight-derived gas, it is needed to achieve high efficiency and low cost production of hydrogen from sunlight irradiation by water splitting. A new effective solar to gas conversion system with multiple DC/DC converter and electrolyzer sets connected with concentrator photovoltaic (CPV) modules have been accomplished in the University of Miyazaki, Japan. The daily high efficiency of solar to hydrogen (~17% on sunny day and ~15% on cloudy day) was obtained from the system by using high efficiency CPV modules connected with multiple converter and electrochemical cells.
Fluctuation of the atmospheric parameters substantially impacts the energy yield of multi-junction cells. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Super-multijunction solar cells were proposed, and optimization work was done. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions.
To obtain high efficiency in crystalline silicon thin film solar cells, it is necessary to obtain novel light trapping structure, which has not only high light scattering effect and small etching margin but also has low surface recombination. Our previous calculation results revealed that silicon nanowires (SiNWs) with submicron diameters have large light scattering effect. In this study, to verify the simulation results, SiNWs with submicron diameters were successfully fabricated by metal assisted chemical etching (MACE) method with silica particles as etching masks. Optical properties and minority carrier lifetime of the obtained structures were measured, and these results suggest that SiNWs with submicron diameters are effective for both of light trapping and suppression of surface recombination.
We investigated improvement of n-bifacial Passivated Emitter and Rear Totally diffused (PERT) solar cell by selective emitter (SE) using non-mass separation type (NMS) ion implantation. Cell efficiency is obviously increased by introducing SE structure. Additionally, efficiency was increased with the increase in the sheet resistance of lightly doped layer, and the gain of η by SE structure was 0.4% absolute. Combining SE and other optimization of cell processes, we have achieved 21% efficiency. This result clearly shows the effectiveness of SE using the NMS ion implanter and the feasibility of high efficiency low cost solar cell using our cell process.
We studied the performance and stability of DSSCs with the electrolyte solution which gelated with a low-molecular
gelling agent. Maximum power, short circuit current and open circuit voltage decreased as the gelling agent concentration
in electrolyte was increased. However, there was no drastic difference between the performance of DSSCs with gel and
sol state electrolytes. In addition, the performance of a DSSC with gel solution was kept for a long time at low temperature.
It is supposed that leakage and volatilization can be prevented by adding small amount of low molecular gelling agent
without decreasing the performance of the DSSC.
Indoor and outdoor power generation characteristics of InGaP // Si spectrum splitting solar cells were measured. In indoor measurement using solar simulator, a conversion efficiency of 28.6% and 30% was achieved at 1 sun and 5.7 suns, respectively. For outdoor measurements, on sunny days, the average efficiency per day is 25%. When the loss of the solar irradiance caused by the measurement box is corrected, the conversion efficiency is 30%. Daily total output is roughly proportional to the accumulated solar irradiance on a daily basis, and the average efficiency of 27% was demonstrated.
Reduction of optical reflection loss at the intermediate region for the mechanically stacked multi-junction solar cells is discussed. The top and bottom substrates are bonded using an adhesive dispersed with indium-tin-oxide particles. Transparent and conductive Indium-gallium-zinc-oxide (IGZO) and tin-dioxide (SnO2) layers are inserted between the adhesive and the substrates as anti-reflection layers. The optical reflectivity of GaAs and Si stacked sample well decreased by the insertion of 130-nm-thick IGZO layers. 157-nm-thick SnO2 layers also decreased the optical reflectivity of stacked sample. High effective absorbency of bottom substrates for the light incident angle between 0 and 50 was achieved for GaAs/Si stacked sample with IGZO layers.