District Heating Water Research Articles

Use of a low-cost phase change material emulsion in de-centralized thermal energy storage for district heating network enlargement

A detailed heat transfer model of a storage tank was developed to assess the performance of a district heating system with de-centralized storage solutions. The performance of two cases, with water and a low-cost phase change material emulsion as storage medium was analysed for 40 residential buildings in Zaragoza (Spain). The chosen configuration is a typical cylindrical tank with internal coils through which the district heating water flows for the charge and discharge. This decentralized thermal energy storage unit reduced peak demand and mass flow in the network, allowing additional buildings to connect to a saturated grid. Four configurations were examined: (1) A 180 m3 water tank reduced district heating power demand by 36.6 %. (2) The same 180 m3 tank using the emulsion required fewer coils, lowering costs, stabilizing TES temperature, and increasing water supply temperature, thus extending system lifespan. (3) A 130 m³ tank with the emulsion offered operational characteristics similar to the water tank, with economic and structural benefits from reduced size. (4) An 180 m³ optimised tank with the emulsion achieved a 38.8 % reduction in district heating power demand. These configurations demonstrate the efficiency and cost-effectiveness of using phase change material emulsion in thermal energy storage systems.

District heating utilizing waste heat of a data center: High-temperature heat pumps

Data centers are energy-intensive facilities with substantial low-grade waste heat. High-temperature heat pumps can be critical in boosting the data center’s waste heat for district heating, improving the system-level energy efficiency of data centers, and reducing CO2 emissions in district heating. This study built thermodynamic models to assess high-temperature heat pumps with six configurations using low global warming potential refrigerants to supply heat up to 120 °C. The heat pump configurations include single-stage or two-stage cycles with advanced components, such as internal heat exchanger, economizer, flash tank, or parallel compressor. The refrigerants include R1234ze(Z), R1233ed(E), R1224yd(Z), R600, and R600a, and R245fa is used as a reference. A case study was carried out to recover the waste heat from the Frontier high-performance computing data center and provide hot water for district heating at the US Department of Energy’s Oak Ridge National Laboratory campus. The optimized performance of high-temperature heat pumps is characterized with various effectiveness of internal heat exchangers, and the operating parameters of economizer or flash tank, as well as their combination. The results show that the configurations of two-stage cycles with internal heat exchanger + flash tank and internal heat exchanger + economizer/parallel-compressor provide the highest coefficient of performance under scenarios of the maximum allowable value and a fixed value (0.3) of the internal heat exchangers’ effectiveness, respectively. R1234ze(Z) and R600a are the most promising refrigerants, considering trade-offs between the coefficient of performance and the volumetric heating capacity. The single-stage cycle with internal heat exchanger + economizer/parallel-compressor using R1234ze(Z) is recommended for utilizing Fronter’s waste heat in district heating. A one mega-watt high-temperature heat pump will reduce 33,100–33,200 metric tons of CO2 emission annually, corresponding to 85.4 %–85.6 % of equivalent CO2 emissions from natural gas boilers. This study provides good guidelines for designing and deploying high-temperature heat pumps to support sustainable data centers and decarbonize district heating in the US.

Energy and economic analysis of flue gas heat recovery systems improving the energy efficiency of gas cogeneration units

This paper presents the results of energy and economic analyses of a gas-fired CHP plant. The analyses were carried out for three variants of technical solutions to improve the thermal efficiency of the CHP plant. The fuel combustion process involves the generation of a large amount of heat. In addition, the heat is dissipated to the surroundings through the exhaust gas with a high temperature of 300÷350 °C. Therefore, to limit heat loss, a heat recovery system is used, which removes heat from the engine body and from the hot exhaust gases through a heat exchanger, allowing the exhaust gases to be cooled to a temperature of 100÷120 °C. In addition, heat is recovered from the oil cooling process and intercoolers. The recovered heat is usually supplied to the heating water in the district heating network. For the calculations, the temperature distribution of the return water from the district heating network during the year was assumed. First, the benefits of adding an additional economizer to pre-heat district heating water were analysed, Next, a heat recovery system was simulated to cool the flue gases to lower temperatures using an absorption heat pump or a compressor heat pump.

Carbon-Free Heat Production for High-Temperature Heating Systems

The article presents a new carbon-free heat production technology for district heating, which consists of a combined heat and power generation fuel cell (FC CHP) with CO2 capture and a two-stage cascade high-temperature heat pump (TCHHP). The FC generates heat and electricity, the latter being used to drive the compressors of the TCHHP. During the winter period, the water temperature achieved can occasionally be too low, so it would be heated up with hydrogen gas boilers. The hydrogen would be produced by reforming natural gas, synthetic methane, or biogas. The results are presented with natural gas utilization—the ratio between the obtained heat flow transferred directly to the water for district heating and the input heat flow of natural gas. In the case of a return water temperature of 60 °C and district heating temperature of 85 °C, the TCHHP, whose heat source is groundwater, achieves plant efficiency of 270.04% in relation to the higher heating value (HHV) and 241.74% in relation to the lower heating value (LHV) of natural gas. A case with a TCHHP whose heat source is low-temperature geothermal water achieves a plant efficiency of 361.36% in relation to the HHV and 323.49% in relation to the LHV.

Design of district heating system with dual-temperature supply based on thermal energy discretisation and matching method

As heating requirements continue to evolve and heating technology advances, radiators and radiant floor heating devices, which require different supply-water temperatures, have gradually become the two main heating terminals in district heating systems. However, the primary return-water temperature of current solutions based on heat exchangers and heat pumps is relatively high and fails to recover waste heat from the combined heat and power plants, owing to improper use of heat sources. To address this issue, a heat exchanger network optimisation method—the thermal energy discretisation and matching method—was used to design a dual-temperature water district heating system consisting of two absorption heat pumps and two heat exchangers. The primary supply water flows through the two generators, two heat exchangers, and two evaporators of the absorption heat pumps. The radiator and floor heating return water was divided into two parts and heated by the heat exchanger and the absorber and condenser of the absorption heat pump, respectively. The performance of the designed heating station was evaluated using the coefficient of performance and the primary return-water temperature and compared with that of other district heating systems. The changes in the district heating system's configuration when the boundary conditions changed were investigated. Results showed that (1) the designed system based on thermal energy discretisation and matching method -had the lowest primary return-water temperature of 18.8 °C and the highest coefficient of performance of 5.62. (2) When the low-temperature load ratio changed from 0% to 100% while maintaining a constant total load, the system's form changed regularly. (3) When the primary supply-water temperature increased, the heating station's form remained unchanged and the capacity of the heat exchange equipment changed regularly. (4) When the secondary supply and return-water temperatures increased, the evaporation temperature of the absorption heat pump increased, resulting in a change in the system's form. This study provides valuable insight into the design of district heating systems that supply dual-temperature water under various design conditions.

Environmental Assessment of Hellisheidi Geothermal Power Plant based on Exergy Allocation Factors for Heat and Electricity Production

The Hellisheidi geothermal power plant, located in Iceland, is a combined heat and power double-flash geothermal plant with an installed capacity of 303.3 MW of electricity and 133 MW of hot water. This study aimed to elucidate the environmental impacts of the electricity and heat production from this double-flash geothermal power plant. In this vein, firstly, the most updated inventory of the plant was generated, and secondly, a life-cycle assessment approach based on the exergy allocation factor was carried out instead of applying the traditionally used allocations in terms of mass and energy. The functional unit was defined as the production of 1 kWh of electricity and 1 kWh of hot water for district heating. The life-cycle stages included the (i) construction, (ii) operation (including abatement operations and maintenance), and (iii) well closure of the geothermal plant. All of the life-cycle stages from construction to dismantling were considered. Finally, the results on the partitioning of the environmental impact to electricity and heat with exergy allocations showed that most of the impact should be charged to electricity, as expected. Furthermore, the distribution of the environmental impacts among the life-cycle stages determined that the construction stage was the most impactful for the electricity and heat production. This result was attributable to the large consumption of steel that was demanded during the construction of the geothermal power plant (geothermal wells, equipment, and buildings). Impacts due to the abatement stage demonstrated that this stage satisfactorily reduced the total impact attributed to the three life-cycle stages of the geothermal power plant.

Derivation of Corrosion Depth Formula According to Corrosion Factors in District Heating Water through Regression Analysis.

In order to predict the corrosion depth of a district heating pipeline, it is necessary to analyze various corrosion factors. In this study, the relationship between corrosion factors such as pH, dissolved oxygen, and operating time and corrosion depth was investigated using the Box-Behnken method within the response surface methodology. To accelerate the corrosion process, galvanostatic tests were conducted in synthetic district heating water. Subsequently, a multiple regression analysis was performed using the measured corrosion depth to derive a formula for predicting the corrosion depth as a function of the corrosion factors. As a result, the following regression formula was derived for predicting the corrosion depth: "corrosion depth (μm) = -133 + 17.1 pH + 0.00072 DO + 125.2 Time - 7.95 pH × Time + 0.002921 DO × Time".

Dynamics of large-scale bubbling fluidized bed combustion plants for heat and power production

Bubbling fluidized bed combustion (BFBC) plants for combined heat and power (CHP) production have traditionally been dispatched under slow load changes. As the amount of variable renewable electricity increases in energy systems worldwide, knowledge regarding the transient capabilities of the gas and water-steam sides of BFBC plants is required. The aim of this work is to investigate the dynamic performance of large-scale BFBC plants when accounting for both the gas and water-steam sides. To do so, this paper presents a dynamic model of BFB-CHP plants that result from connecting a model of the gas side to a process model of the water-steam side. The plant model output is validated by comparisons with operational data measured in a 130-MWth BFBC plant that produces electricity, district heating (DH) water and steam for industrial clients. The validation shows that the model can satisfactorily describe both multi-load steady-state operation and load transients. The simulation results highlight the fact that the water-steam cycle achieves stabilization more rapidly after changes in the DH line and steam delivered to customers, as compared to changes in the combustor load. The timescales of the plant outputs for different changes have been calculated, with stabilization times ranging from 2 to 15 min for the power production versus 2–25 min characterizing the DH production. Compared to the stabilization times of the gas side, the water-steam side is an order of magnitude slower, thereby limiting the transient operation capabilities of BFB-CHP plants.

Influence of temperature of return district water on the performance of a backpressure steam turbine installation

The district heating systems supply heat to a wide range of consumers. In the heat source of such systems, highly efficient technologies are used for the combined production of electrical and thermal energy mainly based on steam turbine installations with backpressure turbines or turbines with adjustable steam extractions. Combined production leads to a reduction in fuel consumption (fuel saving) compared to the separate production of the two energy products. The fuel saving resulting from cogeneration reduces carbon dioxide emissions. Combined production affects the amount of fuel saved, leading to a reduction in emitted emissions, both the size of the heat load realize to consumers and the temperature of the water that enters from the return pipeline of the district heating systems into the heat source. In backpressure steam turbine installations, the district water is heated by the steam that enters the boiler-condenser, and in steam turbine installations with adjustable steam extraction, it is heated in a district heater by steam extracted from the turbine. The purpose of this paper is to study the influence of the temperature of return district heating water on the performance of a backpressure steam turbine installation for cogeneration.

Modeling and Simulation of Solid Oxide Fuel Cell Integrated with Anaerobic Digester, Thermal Storage Unit and Solar Collector: A Net Zero Emission System

Energy production from clean and green sources is one of the eminent challenges to mankind. Overall, all industrial sectors contribute to CO2 emission, but the energy production sector is a major contributor. In recent years, CO2 emissions from the energy sector have increased by 1.7%. Therefore, the development of alternative energy production sources is a pivot for researchers. In this regard, the fuel cell has been a promising technology but still accompanied by the release of greenhouse gasses but relatively lower than that of fossil fuels. The integration of the fuel cell to the biogas has been a promising factor to reduce emissions. This study contributes to the same by producing a self-sustaining biogas-fuel cell multigeneration system for cold areas. Mathematical modeling of all complements of the system, i.e., anaerobic digester, solid oxide fuel cell, solar collector, and thermal storage system, is provided. MATLAB/Simulink environment is used for simulation of the system. The proposed system will use an anaerobic digester for methane production. Hence, produced methane will be used to power solid oxide fuel cell. The electricity of the fuel cell will power the residential place, and the thermal potential of the exhaust will be stored. In daylight, the solar thermal potential will be utilized for district heating. In the absence of solar light, stored thermal energy will be used for district heating and hot water supply. Additionally, the CO2 emitted from the system will not be released into the environment but stored for industrial purposes. The best area of application of the proposed system is cold areas such as Switzerland.

Droop control for district heating networks: Solution for temperature control of multi-energy system with renewable power supply

In a district heating system, the flow of heated water transfers enthalpy from the generation side to the load side. It is thus critical to maintain the supply temperature at a desired level. However, with the rise of renewable energy sources in the district heating network, the temperature of the heated water tends to be disturbed. This calls for a deployment of thermal energy storage. To effectively exploit the flexibility of thermal energy storage, the relationship between the temperature of the district heating water and the power sharing with other thermal energy resources is considered in this paper. Based on this relationship, the concept and realization of a droop control for the district heating network is proposed. It is shown how the droop characteristic between the temperature of the heated water and the power sharing of the thermal energy storage for the district heating network is developed in analogy to the droop control used in an electrical dc network. The droop characteristic so supports the control of a district heating network. The theoretical considerations are complemented by application to a residential district heating network model to illustrate the functionality of the control. The studies are conducted with the program PSCAD, which is otherwise typically used for simulating electromagnetic transients.

Performance analysis of heat pumps with zeotropic mixtures at different load conditions

Heat pump cycles with zeotropic mixtures as working fluids and temperature glides on both heat sink and heat source sides are analyzed. Five binary zeotropic mixtures, selected based on previous studies in the literature, are examined. The cycles are initially optimized for each binary mixture at three different heat source temperature glides (10–30 K) and fixed heat sink temperature glide (30 K). The variation in the heating COP of the optimized designs is then computed and compared for five different part load scenarios down to 40% of the design point heating load. The design point optimization is performed for different single and double-compression cycle layouts, based on which the standard vapor compression cycle with a suction gas heat exchanger is adopted as the most economical option for the rest of the study. The part load cycle simulations demonstrate that, when the heat source temperature glide is large, two of the studied part load scenarios can lead to increased COP; these are when the reduction in the heating load is due to (1) an increase in both the heat sink (e.g. district heating water) inlet and heat source (e.g. process water) outlet temperatures, and (2) a decrease in both heat sink and source flow rates. Moreover, it is observed that different mixtures show different levels of sensitivity to a decrease in heating load. Among the studied mixtures, DME/Iso-Pentane while outperforming others in all studied cases, has the most unfavorable trend at part load, hence losing its benefit margin over other candidates.

Study of Two-Phase Microchannel Heat Sink Fabricated by A.M. Technology for Energy Reuse in the Electronic Device

Data centers’ electricity energy consumption accounts for 1% of global electricity demand and 0.3% of all global CO2 emissions. Energy reuse as a core of a net zero carbon data center, as a macro goal benefiting to mankind, needs micro innovations from thermal engineers to reclaim the distributed and low-grade thermal energy from diversified electronic equipment. This article presents the attempt to combine the advantages of high-density heat transferring technology by two-phase microchannels and the agility of Additive Manufacturing (A.M.) technology into a heat sink by which thermal energy can be collected in premium quantity and quality. The heat sink prototype adopted the two-layer microchannel design by leveraging the unique capability of A.M. technology to form complicated spatial geometric features, such as the functional channel profiles with diverged cross-sections along the flow direction, intermittent channels, and curved channels. It was fabricated at one-time processing by AlSi10Mg powder SLS/SLM, had an exterior base area of 25 cm2, and interior micro-fins with a minimal thickness of 0.2 mm and fin pitch of 0.38 mm. A test rig had been built to validate the thermal dynamic and hydraulic performance of this microchannel heat exchanger working with HFE 7100 as the coolant. The heat flux under certain wall superheat and pressure drop catches the equivalent grade of microchannels made by conventional micro-cutting approaches on copper or aluminum. The maximum inlet coolant temperature could be elevated up to 60.0 °C with less than 90.0 °C CPU case temperature, which provides the feasibility of high-grade heat recovery. The test results present the promising prospects of this design and A.M. technology in the field of two-phase microchannel heat exchanging, by which the electronic devices in megawatt hyperscale data center can be changed from energy consumers to energy generators for the greenhouse, district heating, and hot water system.

Case Study of Using the Geothermal Potential of Mine Water for Central District Heating—The Rožná Deposit, Czech Republic

This paper analyzes the possibility of using the thermal energy of discharged environmentally friendly mine water for the heat supply of a selected locality. There are few cases of industrial use of geothermal water in the Czech Republic, but mine water has never been the source. Based on this fact, an analysis of the usability of mine water at the Rožná I Mine was carried out. The analysis showed that the energy output of this pumped water was sufficient for the selected location of the municipality of Dolní Rožínka, where long-term annual average consumptions are at a level of 4350 GJ. The theoretical maximum output of this source is calculated as 837.4 kW; therefore, it exceeds the output required to satisfy the energy needs of this location several times over. Based on this input information, a technical and economic model of the heating system installation project was developed with three options. The case study aimed to find and propose an optimal alternative solution to replace the current unsatisfactory state of heat supply in the village of Dolní Rožínka. In the final part of this paper, the most optimal option is identified by a comparative method, which replaces the existing central district heating based on the production of heat energy from natural gas, i.e., fossil fuels. This study was motivated by a strategy to replace fossil energy sources with renewable energy sources wherever conditions are suitable.

Integrated development of low-temperature thermal waters

The research concerns the efficient development of low-potential thermal waters in the East-Ciscaucasian artesian basin. A technology is proposed for the integrated development of low-temperature thermal water, using its heat potential for district heating and hot water supply, as well the water itself for various water management purposes, with its quality previously brought to the standards of drinking water. Various chemical water treatment units are suggested for this purpose, with their design and technological features being formed depending on the quality of the source water. The system will enable the maximization of the resource potential of the geothermal well and its all-year-round operation. The paper shows the efficiency of using the potential of geothermal energy resources in energy- biological complexes. The geothermal-biogas technology with the integrated application of thermal waters for various needs is presented. Such thermal water utilization provides for the most efficient use of its thermal potential with a decrease in temperature to a value close to the ambient temperature.

Experimental assessment of the condenser at off-design modes

The article presents the results of thermal tests of the condenser of the steam turbine PT-25-90 UTZ during off-design operation. An increase in the pressure in the condenser for the possibility of heating the district heating water is considered. The pressure in the condenser was artificially deteriorated both by lowering the vapor pressure of the ejector and by reducing the flow rate of the cooling water. The maximum pressure was 70.8 kPa. The maximum exhaust steam temperature was 154 °C. The heat transfer coefficient of the condenser has deteriorated by 75% in comparison with the design mode of operation.

Effect of Imidazole as Corrosion Inhibitor on Carbon Steel Weldment in District Heating Water.

Many research studies have been conducted on the corrosion inhibition performance of imidazole in acidic environments such as in the piping of a petrochemical plant. However, there has been no study on the effect of imidazole in alkaline conditions such as a local district water heating environment. Therefore, in this study, the effect of imidazole as a corrosion inhibitor on carbon steel weldment was investigated in alkaline district heating water. Inhibition efficiency and electrochemical properties were investigated by potentiodynamic polarization test and electrochemical impedance spectroscopy. As the concentration of imidazole increased up to 500 ppm, inhibition efficiency increased up to 91.7%. At 1000 ppm, inhibition efficiency decreased. Atomic force microscopy showed that surface coverage of imidazole at 1000 ppm is lower than that of imidazole at 500 ppm. X-ray photoelectron spectroscopy showed that with 500 ppm of imidazole, the amount of pyrrole type interaction is 4.8 times larger than pyridine type interaction. At 1000 ppm of imidazole, the amount of pyridine type interaction is 3.49 times larger than pyrrole type interaction. Depending on the concentration of imidazole, the ratio of interaction between carbon steel and imidazole affected inhibition efficiency.

A photovoltaic thermal system with a complete contact between water and PV modules suitable for district heating and electric power generation

We studied the dynamic performance of a photovoltaic thermal system with a complete contact between water and PV module at zero reduced temperature (as defined by ASHRAE standard 93–2010). The system gives suitable electric-thermal performances and can be used in district heating and electric power generation systems. All experiments were done in Semnan city (northern part of Iran). The main part of the system involves a mono-crystal photovoltaic module (used also as the heat absorbing plate) attached to a simple water box. Water is used as the coolant fluid with complete contact with the PV module and causes lower temperatures at the module surface. In most cases, the thermal efficiency increases with time while the collector’s heat loss decreases. The maximum and daily averaged reported thermal efficiencies of the PV/T system are 58% and 49%, respectively. The minimum electrical efficiency occurs at noon when the maximum solar radiation occurs, at the same time, it has the highest electrical power output of PV/T system and photovoltaic module. At noon, the electrical efficiencies of the proposed system are 15 to 21.5% more than the conventional photovoltaic modules. The maximum increment of 21.5% is obtained for the water flow of 0.0161 kg/s.

Improving the district heating operation by innovative layout and control strategy of the hot water storage tank

In the district heating (DH) systems, the thermal storage tank is widely used for domestic hot water (DHW) preparation to shave the peak load. Compared to the space heating, the domestic hot water demand exists for all seasons, and the variation of which is more dramatic within short time span. Therefore, the well-controlled storage tank is of great help to stabilize the district heating operation and to improve the overall performance. This study develops an innovative control strategy of the local DHW tank with the heat delivered by DH. According to the dynamic DHW load profile, the controller can switch to the corresponding charging mode to obtain the optimal performance without violating any requirements. A smart-metered multi-storey building in Denmark is selected as the reference case for DHW loop renovation. The proposed control strategy is applied in two new system layouts with the external heat exchanger and the micro heat pump by numerical models. The stratifications of the storage tank, the stability of the DH operation, and the DH return temperatures of the three scenarios are investigated as the key performances. The results show that, to cover the same DHW demand, the total energy consumption of the three scenarios show little difference. However, the proposed charging method of the DHW tank can limit the peak load for DH. Moreover, the resulting DH return temperature can be lowered by 4.9–16.4 °C and the DH water flow can be saved by 8%–28% compared to the reference case.

Model research of the energy efficiency of a cogeneration backpressure steam turbine installation

In the contemporary district heating systems (DHS) heat energy for the customers is generated by cogeneration method, which leads to the saving of primary energy resources compared to the separate production method. The most widespread technology for combined production is based on steam turbine installations with adjustable steam extraction and backpressure steam turbine. In these technologies district heating water is heated to the required temperature either in district heaters in case of steam turbine with adjustable steam extractions or in boiler-condenser in case of backpressure steam turbine installations. The temperature of the district heat water at the inlet of the CHP installation depends on the mode of operation of the DHS. The heat load, distributed to consumers, is regulated at the heat source (CHP installation) by temperature and flow rate of the district heating water, mainly following the change in climatic factors. Current study presents the development of a simulation model of existing CHP backpressure steam turbine. The object studied is a backpressure steam turbine type SST-300 CE2L/V36S. Presented are results from the validation of the simulated model with data from the design documentation. The model has been used to study the energy efficiency of a steam turbine installation based on multivariate simulation calculations. The results obtained relate the energy efficiency indicators of CHP backpressure steam turbine with the factors that characterize the mode of operation of the district heating system.