• Energy Research

Abstract In this study, the summertime outdoor heat stress hazard and heat disorder risk (HDR) were simulated, and their changes under several heat island mitigation scenarios were quantitatively evaluated. Gridded maps that included the 23 wards of Tokyo (23 Tokyo) were produced with 1-km horizontal resolution for consecutive summer months, facing extremely hot weathers, in 2010. The wet-bulb globe temperature (WBGT) was adopted as an outdoor heat stress indicator. Numerical simulations were applied to evaluate the effects of several heat island mitigation scenarios—including building greening (GR), high-albedo coating (ALB), and roof-level emissions of all waste heat by air conditioning (WH)—on the WBGT and HDR for 23 Tokyo. Results showed no cases that effectively lowered the outdoor WBGT and HDR for the studied region. Conversely, the ALB case increased the daily maximum WBGT by an average of 0.6 °C (maximum of 1.7 °C) over all the grids. This increase induced a 1.4 (3.3) times higher HDR in the area average (at the maximum grid) compared to a control case with no mitigation scenario. Hence, heat island mitigation methods reflecting solar radiation could potentially lead to an increase in the outdoor heat stress hazard and risk to a large extent in urban areas.

One of the detrimental effects caused by the urban warming phenomena is the increase of energy consumption due to the artificial air-conditioning of buildings in summer. In greater Tokyo, the temperature sensitivity of the peak electricity demand reaches up to 3%/°C in recent years, and about 1.5 GW of new demand is required as the daily maximum temperature increases by 1.0 °C. This huge demand for summer electricity is considered to be one of the common characteristics of big cities in Asian countries. In order to simulate this increase in cooling energy demands and to evaluate urban warming countermeasures from the viewpoint of buildings' energy savings, a numerical simulation system was developed adopting a new one-dimensional urban canopy meteorological model coupled with a simple sub-model for the building energy analysis. Then, the system was applied to the Ootemachi area, a central business district in Tokyo. Preliminary verification of the simulation system using observational data on the outdoor and indoor thermal conditions showed good results. Simulations also indicated that the cut-off of the anthropogenic heat from air-conditioning facilities could produce a cooling energy saving up to 6% with the outdoor air-temperature decrease by more than 1 °C in the summer urban canopy over Ootemachi area.

AbstractAccurately predicting weather and climate in cities is critical for safeguarding human health and strengthening urban resilience. Multimodel evaluations can lead to model improvements; however, there have been no major intercomparisons of urban‐focussed land surface models in over a decade. Here, in Phase 1 of the Urban‐PLUMBER project, we evaluate the ability of 30 land surface models to simulate surface energy fluxes critical to atmospheric meteorological and air quality simulations. We establish minimum and upper performance expectations for participating models using simple information‐limited models as benchmarks. Compared with the last major model intercomparison at the same site, we find broad improvement in the current cohort's predictions of short‐wave radiation, sensible and latent heat fluxes, but little or no improvement in long‐wave radiation and momentum fluxes. Models with a simple urban representation (e.g., ‘slab’ schemes) generally perform well, particularly when combined with sophisticated hydrological/vegetation models. Some mid‐complexity models (e.g., ‘canyon’ schemes) also perform well, indicating efforts to integrate vegetation and hydrology processes have paid dividends. The most complex models that resolve three‐dimensional interactions between buildings in general did not perform as well as other categories. However, these models also tended to have the simplest representations of hydrology and vegetation. Models without any urban representation (i.e., vegetation‐only land surface models) performed poorly for latent heat fluxes, and reasonably for other energy fluxes at this suburban site. Our analysis identified widespread human errors in initial submissions that substantially affected model performances. Although significant efforts are applied to correct these errors, we conclude that human factors are likely to influence results in this (or any) model intercomparison, particularly where participating scientists have varying experience and first languages. These initial results are for one suburban site, and future phases of Urban‐PLUMBER will evaluate models across 20 sites in different urban and regional climate zones.

Abstract There is growing interest in the utilization of unused, but possible, energy sources to reduce carbon-dioxide emissions and fossil-energy consumption, and especially to comply with the Kyoto Protocol which came into effect in 2005. Detailed considerations of plant location, land use and life cycle analysis, however, have not yet been fully estimated with a view to confirming the advantages of the new energy-source usage. A model for heat energy from river water and treated sewage water, and waste-heat energy from municipal solid-waste incineration plants was built and applied to the Tokyo urban area in Japan, considering the spatial and time-related distribution of demands and supplies, the shapes of buildings in the demand area, and life-cycle analysis. The model selected areas were those which should use these energies without prejudice, and sometimes the areas were far from the energy-source point. The reduction of carbon-dioxide emissions resulting from new energy-sources was about 8% of the reduction target for Tokyo in 1990. The model was able to precisely evaluate the new energy-usage, using data from both supply and demand sides.

We present an assessment of installing a regional heating and cooling system in the Nishi(West)-Shinjuku area of Tokyo, Japan. In this assessment, we estimate the CO2 payback–time, when air source heat–pumps (ASHP) are replaced with a ground–source heat–pump (GSHP) system. We calculate CO2 emissions from transportation of the cooling tower, materials for the underground heat exchanger, and the digging loads and transportation loads incurred when the GSHP system is installed to replace the air source cooling system. The total CO2 emission from the installation of the GSHP system was estimated to be 67,701t-CO2, with 87% of the CO2 emissions resulting from the digging process. CO2 emissions from the operation of the GSHP system were estimated from the total energy-efficiency of the system and the heating and cooling demand in Nishi-Shinjuku area. Using the GSHP system, 33,935t-CO2 would be emitted per year. We estimate that using the GSHP system would result in a reduction of 54% of the CO2 emissions, or 39,519t-CO2 per year. From these results, the CO2 payback–time for replacing the conventional ASHP in the 1 km2 studied region with the GSHP system is assessed to be 1.7 years.

This paper describes the effects of the installation of various countermeasures against urban heat-island (UHI) and energy-saving measures on UHI and global warming. A UHI and energy-consumption simulation model was developed by combining the one-dimensional meteorological canopy and building energy use models; further, the proposed model was expanded to evaluate the year-round air temperature and annual energy consumption. The simulation results showed that the humidification and albedo increase at building-wall surfaces reduced the total number of hours for which the air temperature was more than 30 °C during the daytime by more than 60 (h) per year. The UHI countermeasures reduced the annual energy-consumption despite causing a small increase during the winter period. However, they may result in certain unfavorable conditions for pedestrians. Energy-saving measures, on the other hand, reduce the total number of hours for which the air temperature is more than 30 °C by only a few hours per year. Thus, we demonstrate the effectiveness of the UHI countermeasures and measures against global warming by extending the calculation period from summer to an entire year.

Abstract The electricity consumption (EC)–outdoor near-surface air temperature (T) relationships in the Tokyo Metropolitan Area were revealed by EC data at 1290 substations with high temporal and spatial resolutions provided by an electric power company. Temporal–spatial variation in the sensitivity of EC to T (ΔEC/ΔT) was seen. The daytime ΔEC/ΔT values in business and commercial areas were larger than in residential areas. The ΔEC/ΔT values in business substations showed larger scatter than those in residential substations. One source of this scatter was building size, which affects the constitution of AC systems installed in the buildings. This study is the first to describe such detailed ΔEC/ΔT data (including uncertainties). The work will aid development of efficient energy management systems and the results can be used as a benchmark and input data to improve urban climate and building energy models.