Ryozo Ooka

The University of Tokyo, 白山, Tōkyō, Japan

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Publications (190)154.15 Total impact

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    Wonjun Choi · Ryozo Ooka
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    ABSTRACT: To interpret thermal response tests (TRTs), analytical models that assume constant heat flux from the source are widely used because of their simplicity. However, in actual field conditions, the constant heat flux assumption is violated by the heat exchange between the above-ground TRT setup and outdoor environment. This results in perturbations in the temperature response and causes fluctuations in estimation and consequent estimation errors in the interpretation of TRTs. For a better design of experiments and obtaining quality data from a TRT, a systematic analysis of the disturbance factors is important. In this study, we developed an analytical model that describes the heat exchange in an above-ground TRT setup. On the basis of this model, a parametric study and sensitivity analysis were conducted in a systematic manner using disturbance-related parameters, such as test settings (heat injection rate and flow rate), above-ground connecting circuit parameters (insulation thickness, length, and radiation absorptivity), temperature of fluid, and weather conditions (solar irradiation, environmental temperature, and wind velocity). The above-ground circuit length and parameters related to radiative heat transfer showed the highest sensitivity coefficients. Based on the results, some suggestions are provided for experimenters on designing TRT setups and conducting TRTs to obtain quality data.
    Full-text · Article · Jan 2016 · Renewable Energy
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    Preview · Article · Jan 2016 · Energies
  • Shintaro Ikeda · Ryozo Ooka
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    ABSTRACT: Co-generation systems (CGS) and energy storage equipment, such as batteries and thermal energy storage (TES), have become increasingly important recently for improving energy efficiency and for adjusting or reducing peak loads of energy systems. However, optimization of operating schedules of such energy storage equipment is difficult due to its parameter dependency. In addition, a nonlinear characteristics of recent heat-source machines have made the problem more difficult. Therefore, we applied an efficient optimization method, εDE (epsilon constrained differential evolution) for single objective optimization, to minimize operating costs. Moreover, we proposed εMODE (epsilon constrained multi-objective differential evolution) to solve the complex trade-off problem (costs vs primaryenergy consumption). Each result showed that these methods could provide the optimum solution in a practical time, even if the problem had a lot of decision variables that were nonlinear. In particular, εMODE could carry out a lot of non-dominated solutions without concentrating on a certain position and generating dominated solutions.
    No preview · Article · Jan 2016 · Journal of Environmental Engineering (Transactions of AIJ)
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    Wonjun Choi · Ryozo Ooka
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    ABSTRACT: The approximated infinite line source (ILS) model is widely used to interpret thermal response tests (TRTs). It assumes a constant heat flux from the source. However, this assumption is violated in real field conditions by the heat exchange between the circulating fluid and the outdoor environment in an above-ground TRT setup. This results in a fluctuating behavior of sequential estimation and estimation error. In this study, we quantitatively examined the effect of disturbance from outdoor environment on This, especially when TRTs are interpreted by the ILS model, using numerical methods. An analytical model that takes disturbance into account was incorporated as the boundary condition of a numerical model. Using typical synthetic weather data of different seasons and 36 cases of measured weather data, numerical TRTs were conducted and interpreted. Some characteristic behavior of interpretation related to weather conditions was explained and changes in error range with testing duration were analyzed to clarify the applicability and limitation of the interpretation using the ILS model. The results showed that at least 60 h of TRT is required to obtain results within the error range of +5% compared with the reference case. Additionally, some practical suggestions regarding conducting and interpreting TRTs are provided. (c) 2015 Elsevier Ltd. All rights reserved.
    Full-text · Article · Jan 2016 · Renewable Energy
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    ABSTRACT: Thermo active building systems (TABS) have been applied in office buildings as a promising energy efficient solution in many European countries. The utilization of building thermal mass helps to provide high quality thermal environments with less energy consumption. However, the concept of TABS is entirely new in Japan. This paper introduces and evaluates TABS under Tokyo weather conditions to clarify the potential of use TABS in Japan. Cooling capacity of thermo active building systems used in an office building was evaluated by means of dynamic simulations. Two central rooms of the office were selected for the analysis. Six water control strategies were studied and two of those were found reasonable and suitable for TABS use in Tokyo. These two strategies are: free-cooling using underground heat exchanger combined with TABS and free-cooling with desiccant dehumidification system. For these two cases, the operative temperature was between 22-27°C during 97∼99% of the occupation time. The operative temperature drift was less than 4°C per day. The pump running time was 7hours per day and the cooling power of the TABS was 36W/m2 floor area. For those free-cooling cases, the average supply water temperature was 20°C, which shows that free-cooling is achievable using underground heat exchangers even considering the temperature increase of the ground during cooling season.
    Full-text · Article · Nov 2015 · Energy Procedia
  • Wonjun Choi · Ryozo Ooka
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    ABSTRACT: Thermal response tests (TRTs) are essential for reliable and accurate design of ground source heat pumps. The approximated infinite line source (ILS) model is widely used to interpret TRTs because of its simplicity. However, in actual field conditions, the constant heat flux assumption of the ILS model is violated in most cases because the above-ground hydraulic circuit, which connects the borehole heat exchanger (BHE) and the TRT rig, is exposed to the outdoor environment. This results in perturbations in the temperature response, occasioned by heat exchange between the circulating fluid and the outdoor environment. This disturbance causes fluctuations in sequential estimation and consequent estimation errors in TRTs. As a first step in investigation of the estimation error in TRTs, this paper reports on the development of an analytical model that considers the heat exchange in the hydraulic circuit connecting the BHE to the TRT rig, and its incorporation into the numerical BHE model. The results of comparison of the integrated numerical BHE model against actual experimental in situ TRT data show very good agreement, with maximum difference and maximum relative error of approximately 0.2K and less than 0.8%, respectively.
    No preview · Article · Nov 2015 · Energy Procedia
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    Marco Perino · Shintaro Ikeda · Ryozo Ooka
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    ABSTRACT: Some types of storage equipment, such as thermal energy storage (TES) and batteries, have recently become increasingly important for peak-load shifting in energy systems. In addition, the sale of photovoltaic (PV) system and battery electric power has also helped minimize operating costs. However, optimizing energy systems is difficult because each machine has multiple combinations of operations, and the objective function and modeling of some practical machines contain transformed nonlinear or non-convex characteristics. Therefore, we adopted the epsilon-constrained differential evolution (ɛ DE), which is categorized as a metaheuristic optimization method, in order to minimize operating costs under nonlinear conditions and various energy system connections. First, we consider the case in which electric power generated from the PV system and battery is provided only to the electric demand. The second case limits the sale of this electric power to the electric grid. The third condition includes the sale of this electric power not only to the grid but also to the electric demand, heat source machinery, and some pumps. We demonstrate that the ɛ DE method efficiently solved this strict constraint optimization problem. Moreover, we confirm that the total-amount purchase system (the second case) is not always suitable when minimizing operating costs because it depends not only on the price of the purchased electricity but also on the price of the sold electricity.
    Preview · Article · Nov 2015 · Energy Procedia
  • Shintaro Ikeda · Ryozo Ooka
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    ABSTRACT: Thermal energy storage (TES) and batteries have recently become increasingly important for peak-load shifting in energy systems. However, optimizing energy systems is difficult because each machine has multiple combinations of operations, and the objective function contains transformed nonlinear or non-convex characteristics. Therefore, we adopted the epsilon-constrained differential evolution (ɛDE) in order to minimize operating costs. We demonstrate that the ɛDE method efficiently solved strict constraint optimization problems on three energy systems: a self-consumption model (Case 1), total amount of a purchased model (Case 2), and a full connection model (Case 3) under 126 case studies. Although 216 decision variables were used under the nonlinear condition, we were able to obtain the optimal solution within a short time period, 16 min on an ordinary PC. Moreover, we proposed a new index “Area rate of prices (ARP)” in order to evaluate the effects of purchased and sold electricity prices on the operating costs. The results showed that when the area rates of purchased price to sold price are higher than 0.2 (ARP > 0.2), Case 1 was superior to Case 2. On the other hand, when the ARP value was less than 0.2, Case 2 was superior to Case 1. Therefore, we can conduct the optimization on everyday practical situations because ɛDE requires low computational cost. Even if the operators cannot conduct the optimization in practical energy management, they can easily determine the operation strategy by calculation of the ARP value. Therefore, the ɛDE and ARP methods have substantial advantages for energy system optimization.
    No preview · Article · Nov 2015
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    Madhavi Indraganti · Ryozo Ooka · Hom B Rijal
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    ABSTRACT: Reports on occupant behavioral adaptation to varying thermal conditions in India are limited. We analyzed the data from our thermal comfort survey of 2787 occupants in 28 office buildings (N= 6048): 13 mixed mode (MM), 14 completely air-conditioned and one naturally ventilated building in Chennai and Hyderabad. Behavioral adaptation was found to be vital for thermal comfort. Occupants adaptively used air-conditioners and or fans in MM buildings during the temperature excursions. Our logistic regression predicts 89.4% and 28.5% of air conditioners to be operating in MM buildings in Chennai and Hyderabad respectively, at 29 °C of outdoor daily mean temperature. Females, young subjects, and people with low body mass index had higher comfort temperatures than males, older people, and obese occupants respectively, with a difference of 0.3 – 1.0 K in various groups, which is significant at 95% confidence interval. In naturally ventilated environments females were comfortable at 28.5 °C and men at 27.8 °C. Females accepted the environments better. Many behavioral control actions were noted throughout the year without much seasonal shifts. Staying in airy place was the most prominent behavioral adaptation all through. This reinforces the need for elevated air movement indoors.
    Full-text · Article · Sep 2015 · Energy and Buildings
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    ABSTRACT: In this study, we dynamically downscaled the Model for Interdisciplinary Research on Climate version 4 (MIROC4h) in August for the present (2001–2010) and the near future (2026–2035). We selected weather data that represent the average weather conditions during 10-year periods among the results of downscaled MIROC4h. Correcting the selected weather data with observations to reduce bias of both regional climate model (RCM) and global climate model (GCM), we constructed a prototype of the near-future design weather data of the 2030s. We conducted building energy simulations using the prototype of design weather data to assess the impact of climate change on energy consumption of a two-story detached house in Tokyo. Under these conditions, total sensible heat load in August increased 26%, and the latent heat load increased 10%.
    No preview · Article · Aug 2015 · Energy and Buildings
  • Shintaro Ikeda · Ryozo Ooka
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    ABSTRACT: Storage equipment, such as batteries and thermal energy storage (TES), has become increasingly important recently for peak-load shifting in energy systems. Mathematical programming methods, used frequently in previous studies to optimize operating schedules, can always be used to derive a theoretically optimal solution, but are computationally time consuming. Consequently, we use metaheuristics, such as genetic algorithms (GAs), particle swarm optimization (PSO), and cuckoo search (CS), to optimize operating schedules of energy systems that include a battery, TES, and an air-source heat pump. In this paper, we used a GA, differential evolution (DE), our own proposed mutation-PSO (m-PSO), CS, and the self-adaptive learning bat algorithm (SLBA), of which m-PSO was the fastest, and CS was the most accurate. CS obtained the semi-optimal solution 135 times as fast as dynamic programming (DP), a mathematical programming method with 0.22% tolerance. Thus, we showed that metaheuristics, especially m-PSO and CS, have advantages over DP for optimization of the operating schedules of energy systems that include a battery and TES.
    No preview · Article · Aug 2015 · Applied Energy
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    Full-text · Conference Paper · Jul 2015
  • Hideki Kikumoto · Ryozo Ooka · Yusuke Arima
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    ABSTRACT: Enhancement of very hot weather conditions in summer due to climate changes at global and local scales can increase mortality through thermal stresses. To estimate the impact of climate changes on the risks of heat disorder (HD) in Japan around the year 2030, we conducted numerical climate simulations using a global circulation model (GCM) and a regional climate model (RCM). The GCM results for the global climate change analysis were provided as initial and boundary conditions, and climate information was dynamically downscaled by the RCM. We carried out simulations of the situation in August over 10-year periods for the present (2001–2010) and near-future (2026–2035) cases. The wet-bulb globe temperature (WBGT) was used as a thermal stress index. The modeled 10-year average WBGT values for the present time agreed very well with observation data. An increase of 1.11 °C (from 24.96 °C to 26.07 °C) in average WBGT from the present to the future cases was predicted based on the data. Changes in HD incidence rates were then evaluated using the obtained results and past statistical data. The average number of people transported by ambulance per day in August was predicted to increase by 63%.
    No preview · Article · Jul 2015 · Energy and Buildings
  • Ryozo Ooka · Shintaro Ikeda
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    ABSTRACT: The increasing popularity of smart energy systems has led to a gradual increase in the importance of thermal energy storage (TES) technology. Thus, the control strategy employed to efficiently take advantage of TES is expected to be very important. In other words, the time schedule, the particular components to be activated, and the amount of charging/discharging have to be appropriately determined. To date, a number of studies have investigated the optimization of TES operations by using optimization techniques. Current methods being used to achieve optimal TES operation are reviewed in this paper.
    No preview · Article · Jul 2015 · Energy and Buildings
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    Hideki Kikumoto · Ryozo Ooka
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    ABSTRACT: Vertical profiles of wind velocity within the urban boundary layer were measured in the Tokyo center district for seven months using a Doppler lidar system. We applied the power law to approximate the measured wind profiles and discussed their approximation accuracy. The power-law index showed a diurnal variation from 0.1 in the daytime to 0.3 in the night; the average of the power-law index is 0.206 for the all observation periods. The approximation accuracy significantly depended on wind velocity and averaging period. The power law could be a good model for high velocity conditions with long averaging time. However, the relative error of the approximation using the power law became more than 20% for cases with 10-minute average in which wind velocity was less than 5 m/s.
    Full-text · Conference Paper · Jun 2015
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    Hang Yin · Ryozo Ooka · Masanori Shukuya
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    ABSTRACT: To elucidate the energy-saving effect of pumps along with the heat-transfer performance of terminal units in heat pump systems, a comprehensive analysis of an assumed chilled-water circuit at two supply water temperatures under four variable-flow control modes was carried out from the viewpoint of available energy, i.e., exergy. Subsequently, based on the operating data, the exergy analysis of a heat pump system was carried out to verify the energy-saving effect after variable-frequency transformation of the chilled water pump. The corresponding results based on the operating data of an actual heat pump system are consistent with those obtained through theoretical analysis.
    Full-text · Conference Paper · Jun 2015
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    Hang Yin · Ryozo Ooka · Masanori Shukuya
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    ABSTRACT: The energy used by HVAC systems accounts for a large proportion of a building's total energy use. Improving the efficiency of an HVAC system would therefore realize significant energy savings. In an HVAC system, operation under maximum load accounts only for a small percentage of the total operating time. Reducing the energy used by the pumping system when the system is under partial load would significantly improve the efficiency of an HVAC system. On the other hand, the supply water temperature of the chilled water circuit is closely associated with the heat-transfer characteristics of the heat-exchanger units. Choosing an appropriate supply temperature would improve the heat-transfer performance of the related components in an HVAC system. Therefore, to clarify the energy-saving effect of the pumps together with the heat-transfer performance of the terminal units, a combined analysis of an assumed chilled-water circuit under four variable-flow control methods at two supply water temperatures were carried out from the viewpoint of available energy, that is, exergy. Subsequently, based on the operating data, the exergy inputs to a real chilled water circuit before and after the variable-frequency transformation of the chilled water pump were compared. The results obtained from the exergy analysis were as follows: (1) Exergy input and exergy consumption under partial load is significantly less than that under full load; (2) The exergy input and exergy consumption with any variable-frequency control is less than that with the throttle-valve control; (3) Among the different types of variable-frequency control, the exergy input is reduced in the following order: constant-pressure control, constant-differential-pressure control, and predictive-system-curve control; (4) The exergy inputs are lower at a supply water temperature of 12°C (53.6°F) than at 7°C (44.6°F); (5) The analysis results based on real operating data are consistent with those obtained through theoretical analysis.
    Full-text · Conference Paper · Jun 2015
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    Wonjun Choi · Ryozo Ooka
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    ABSTRACT: Effective ground thermal conductivity and borehole thermal resistance, which are key parameters in the design of borehole heat exchangers (BHEs), are often determined on the basis of in-situ thermal response tests (TRTs). However, many disturbance factors can affect the accuracy of a TRT, e.g., voltage fluctuations from the power grid and oscillating external environments where a TRT rig is installed. Interpretation of TRT data is often done using the infinite line source (ILS) model, combined with the sequential plot method, because it is not only simple but also provides additional information about the estimation behavior and convergence. However, estimation behavior using the sequential method tends to fluctuate over time because the constant heat flux assumption is always violated as a result of the disturbance factors. As an alternative, a temporal superposition applied analytical model can be used in a recursive curve fitting manner, but this method cannot provide the additional information that sequential method can. In this study, as a solution for interpreting disturbed TRT data and to utilize additional information from the sequential plot method, we proposed an alternative method using a temporal superposition applied ILS model combined with the quasi-Newton optimization method. To verify the effectiveness, the proposed method was applied to in-situ TRTs and the results were compared with those from the conventional method in terms of the estimation stability and convergence speed. The results showed that, compared to the conventional sequential method using the ILS model, the proposed method yielded standard deviations for the effective thermal conductivity and borehole thermal resistance that were at least six times and four times lower, respectively. Moreover, the proposed method was able to achieve about four times faster convergence speeds.
    Full-text · Article · Jun 2015 · Applied Energy
  • Hang Yin · Ryozo Ooka
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    ABSTRACT: In the design calculation of a plate-fin heat exchanger, the convective heat transfer coefficients of the plate and fin are often simplified to the same value. Further, in the calculation of the convective heat transfer coefficient, the Colburn heat transfer factor and Fanning friction factor are generally obtained using empirical equations. In this work, to obtain more precise results, the convective heat transfer coefficients of the plate and fin are defined as independent parameters. The Colburn factor and Fanning factor are obtained according to the results of computational fluid dynamics. The fin height, fin pitch, fin thickness, and fin length are considered as four design parameters. A modified number of entropy production units (Ns) is adopted. Ns due to friction (NsΔP), Ns due to heat transfer (NsΔT), and Ns are considered as three objective functions. The optimal structural parameters of a water-to-water plate-fin heat exchanger applied to an air-conditioning system are obtained using genetic algorithm by single objective optimization and multi-objective optimization.
    No preview · Article · Feb 2015 · Applied Thermal Engineering
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    ABSTRACT: As in many other countries in the world, Japan has witnessed an increased focus on low-energy buildings. For testing different engineering solutions for energy-efficient buildings, a low-energy building was built at the University of Tokyo as an experimental pilot project. In this building, a radiant heating/cooling ceiling panel system is used. However, no standard exists for the in situ performance evaluation of radiant heating/cooling ceiling systems; furthermore, no published database is available for comparison. Thus, this study aims to not only clarify the system performance but also to share our experience and our results for them to serve as a reference for other similar projects. Here, the system performance in relation to its heating/cooling capacity and thermal comfort has been evaluated. The heat transfer coefficient from water to room was 3.7 W/(m(2) K) and 4.8 W/(m(2) K) for heating and cooling cases, respectively. The upward heat flux from the panels was found to be as large as 30-40% of the water heating/cooling capacity; this would translate into heat loss in certain operating modes. Several proposals for reducing the upward heat flux were discussed. The measurements also showed that a category B thermal environment was obtained using the radiant ceiling heating/cooling system.
    Full-text · Article · Jan 2015 · Energy and Buildings