Eco-Friendly System
Modeling of a Fourth-Generation District Heating System
Modeling of an Eco-Friendly Energy Town
Development of a Secondary Fluid System Using CO₂ Hydrate
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이산화탄소- 하이드레이트활용 이차유체시스템 개발
Modeling of a Fourth-Generation District Heating System
◇ Research Background
An indirect secondary fluid system refers to a configuration in which the refrigerant does not directly contact the evaporator of the cooling unit. Instead, a secondary fluid—cooled by the primary system—is circulated to the load side. Typical secondary fluids include carbon dioxide, cooling water, ice slurry, and brine, and recently, gas hydrates have also emerged as a subject of active research.
Gas hydrates are solid crystalline compounds formed from water and gas molecules under specific temperature and pressure conditions, as illustrated below. Compared with other substances used in secondary fluid systems, gas hydrates exhibit exceptionally high latent heat characteristics. Owing to this property, gas hydrates are considered highly effective for cold thermal energy storage. In fact, using gas hydrates as a storage medium can be more efficient than producing low-temperature ice slurry.
The CO₂–hydrate experiment involves controlling temperature variables within the system by employing a brine chiller and a heat exchanger connected to the CO₂ evaporator. The condenser of the refrigeration system is also connected to a heat exchanger coupled with a brine chiller, enabling adjustment of the overall system temperature. Additionally, the performance of the CO₂–hydrate slurry as a working fluid is evaluated using established thermal performance equations and compared with that of water.
◇ Research Background
To improve the utilization efficiency of renewable and unused energy sources, fourth-generation district heating systems aim to replace conventional high-temperature supply water—typically above 100°C—with low-temperature supply water below 60°C. These systems incorporate diverse heat sources to reduce dependence on fossil fuels and expand the use of renewable thermal energy. In Korea, most operating systems correspond to third-generation district heating, with second- and third-generation systems co-existing at many sites.
◇ Research Method – Development of a Borehole Thermal Energy Storage (BTES) Solar System Analysis Model
During the summer, the solar collector unit harvests thermal energy, which is then stored in both a solar thermal storage tank and a borehole thermal energy storage (BTES) unit. During the winter season, the stored thermal energy is extracted and supplied for space heating and domestic hot water. If the stored energy from the solar storage tank or BTES is insufficient, additional geothermal energy and supplemental solar/BTES heat are upgraded using a heat pump and stored in a nighttime thermal storage tank, which subsequently supplies heating and hot water. This study focuses on developing an analytical BTES model that simulates this integrated solar-geothermal thermal energy storage system.
친환경 에너지타운 모델링
Modeling of an Eco-Friendly Energy Town
◇ Research Background
A solar thermal system consists of a collector unit, a storage unit, and a utilization unit.
The collector unit stores solar thermal energy in the working fluid circulating through the collectors.
The storage unit transfers heat from the working fluid circulating in the collector loop to the working fluid circulating in the storage loop via a heat exchanger, and the transferred energy is stored in the thermal storage tank.
The utilization unit supplies domestic hot water and space heating using the thermal energy stored in the storage tank.
◇ Research Results
Graph 1 compares the load-side temperature obtained from the seasonal thermal storage tank between the TRNSYS simulation and the actual seasonal storage system. The red line represents the load temperature predicted by TRNSYS, and the blue line represents the temperature measured from the actual seasonal storage system.
Graph 2 compares the domestic hot water temperature between TRNSYS and the actual measured temperature. The red line indicates the load (domestic hot water) temperature calculated using TRNSYS, and the blue line shows the temperature measured from the actual seasonal storage system.
graph 1
graph 2
중대규모 지중 계간축열 시스템의 해석적 연구
Analytical Study of a Medium-to-Large-Scale Borehole Seasonal Thermal Energy Storage System
◇ Research Background
This study aims to provide fundamental data for borehole seasonal thermal energy storage (STES) systems by performing analytical simulations using TRNSYS. Based on the configuration of the Drake Landing Solar Community system, the study analyzes how borehole depth affects the temperature of the underground thermal storage, as well as the amount of heat transferred to the solar thermal storage tank and to the borehole storage. Through this analysis, basic design and operation data for medium-to-large-scale underground seasonal thermal storage systems are derived and presented.
◇ Research Results
Using TRNSYS and adopting the installation and operating conditions of the Drake Landing System as a reference, this study analyzed the variation in underground storage temperature and the heat transferred to both the solar thermal storage tank and the borehole thermal storage as a function of borehole depth in a medium-to-large-scale STES system.
For Case 2, where the borehole depth is 33 m, the heat transferred to the solar thermal storage tank was 1,738.6 GJ, and the heat transferred to the borehole thermal storage was 1,220.4 GJ, both representing maximum values among the analyzed cases. As the borehole depth deviated from 33 m, these values showed a decreasing trend. Thus, a borehole depth of 33 m is considered the optimal depth for the system within the scope of this study.
발전소 온배수를 이용한 히트펌프 시스템의 이론적 연구
Theoretical Study of a Heat Pump System Using Power Plant Waste Heat Seawater
◇ Research Background
In this study, a heat pump system utilizing power plant waste heat seawater as a heat source was configured for greenhouse heating in Jeju Island. The performance coefficient (COP) of the heat pump was calculated for two cases: when using ordinary seawater and when using waste-heat seawater discharged from a power plant. The system was then analyzed by comparing COP values according to the type of heat source.
The simulation program TRNSYS was used to model the operation and control of the heat pump system. Through this modeling, various operating conditions were examined to derive optimal scenarios depending on the heat source characteristics.
◇ Research Results
Using TRNSYS simulations, this study analyzed the heat pump system driven by power plant waste-heat seawater and compared the COP of the heat pump for different heat sources. Jeju Island meteorological data were used as input, and temperature and operating data for both waste-heat seawater and ordinary seawater were applied to conduct a comparative analysis of each system.
Figure 3 shows the average COP of two heat pumps when seawater is used as the heat source.
Figure 4 presents the COP of the heat pumps when power plant waste-heat seawater is used as the heat source.
For the first heat pump (TYPE688), the COP was 7% higher when using waste-heat seawater compared to ordinary seawater. For the second heat pump (TYPE688-2), the COP was 3.2% higher when waste-heat seawater was used as the heat source.
