Low Temperature Heat Sources Research Articles

Temperature control of a low-temperature district heating network with Model Predictive Control and Mixed-Integer Quadratically Constrained Programming

District heating networks transport thermal energy from one or more sources to a plurality of consumers. Lowering the operating temperatures of district heating networks is a key research topic to reduce energy losses and unlock the potential of low-temperature heat sources, such as waste heat. With an increasing share of uncontrolled heat sources in district heating networks, control strategies to coordinate energy supply and network operation become more important. This paper focuses on the modeling, control, and optimization of a low-temperature district heating network, presenting a case study with a high share of waste heat from high-performance computers. The network consists of heat pumps with temperature-dependent characteristics. In this paper, quadratic correlations are used to model temperature characteristics. Thus, a mixed-integer quadratically-constrained program is presented that optimizes the operation of heat pumps in combination with thermal energy storages and the operating temperatures of a pipe network. The network operation is optimized for three sample days. The presented optimization model uses the flexibility of the thermal energy storages and thermal inertia of the network by controlling its flow and return temperatures. The results show savings of electrical energy consumption of 1.55%–5.49%, depending on heat and cool demand.

Organic thermoelectric devices with PEDOT:PSS/ZnO hybrid composites

Organic thermoelectric devices (OTEDs) are considered one of the most promising thermoelectric devices for harvesting electricity from low-temperature heat sources. Here it is demonstrated that the hybrid composites of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and zinc oxide (ZnO) particles enhance the performances of OTEDs due to the improved electrical conductivity by the bridging role of ZnO particles. The detailed study showed that the addition of ZnO particles to the aqueous solutions of PEDOT:PSS led to the increased viscosity owing to the specific interactions between sulfonic acid groups in PSS and hydroxyl groups (and/or zinc cations) in ZnO. The OTEDs with the PEDOT:PSS/ZnO hybrid composites with 70 wt% of 45 μm-sized ZnO particles exhibited 0.55 nW, which is ca. 7.8 times higher power than the devices with the pristine PEDOT:PSS sheets (0.07 nW). The wearable thermal sensors with the PEDOT:PSS/ZnO hybrid composite films showed fast thermoelectric responses toward heat and cool sources.

Performance Investigation of Solar Organic Rankine Cycle System With Zeotropic Working Fluid Mixtures for Use in Micro-Cogeneration

Abstract Overall, there are numerous sustainable sources of renewable, low-temperature heat, principally solar energy, geothermal energy, and energy produced from industrial wastes. Extended utilization of these low-temperature alternatives has a certain capacity of decreasing fossil fuel use with its associated very hazardous greenhouse gas emissions. Researchers have commonly recognized the organic Rankine cycle (ORC) as a feasible and suitable system to produce electrical power from renewable sources based on its advantageous use of volatile organic fluids as working fluids (WFs). Researchers have similarly shown an affinity to the exploitation of zeotropic mixtures as ORC WFs due to their capability to enhance the thermodynamic performance of ORC systems, an achievement supported by improved fits of the temperature profiles of the WF and the heat source/sink. This paper determines both the technical feasibility and the benefits of using zeotropic mixtures as WFs by means of a simulation study of an ORC system. This study analyzes the thermodynamic performance of ORC systems using zeotropic WF mixtures to produce electricity driven by low-temperature solar heat sources for use in buildings. A thermodynamic model is created with an ORC system with and without a regenerator. Five zeotropic mixtures with diverse compositions between 0 and 1 in 0.2 increments of R245fa/propane, R245fa/hexane, R245fa/heptane, pentane/hexane, and isopentane/hexane are assessed and compared with identify the best blends of mixtures that are able to produce superior efficiency in their system cycles. Results disclosed that R245fa/propane (0.4/0.6) with regenerator produces the highest net power output of 7.9 kW and cycle efficiency of 9.4% at the operating condition with a hot source temperature of 85 °C. The study also investigates the effects of the volume flow ratio, and evaporation and condensation temperature glide on the ORC’s thermodynamic performance. Following a thorough analysis of each mixture, R245fa/propane is chosen for a parametric study to examine the effects of operating factors on the system’s efficiency and sustainability index. It was found that the highest cycle efficiency and highest second law cycle efficiency of around 10.5% and 84.0%, respectively, were attained with a mass composition of 0.6/0.4 at the hot source temperature of 95 °C and cold source temperature of 20 °C with a net power output of 9.6 kW. Moreover, results revealed that for zeotropic mixtures, there is an optimal composition range within which binary mixtures are tending to work more efficiently than the component pure fluids. In addition, a significant increase in cycle efficiency can be achieved with a regenerative ORC, with cycle efficiency in the range 3.1–9.8% versus 8.6–17.4% for ORC both without and with regeneration, respectively. In conclusion, utilizing zeotropic mixtures may well expand the restriction faced in choosing WFs for solar-powered ORC-based micro-combined heat and power (CHP) systems.

Real Air-Conditioning Performance of Ejector Refrigerator Based Air-Conditioner Powered by Low Temperature Heat Source

In this present work, the air-conditioning test performance of an ejector refrigerator-based air-conditioner (ERAC) was proposed. The ERAC was operated as the water chiller to produce the cooling load up to 4.5 kW. The chilled water temperature was later supplied to the fan-coil unit for producing the thermal comfort condition. The cooling water used to cool the condenser was achieved from the cooling tower which was operated under the hot and humid ambient. This is to demonstrate the feasibility of using the ERAC in real working conditions. The cooling load supplied to the air-conditioned space was applied by the air heater. The ERAC could efficiently be operated to produce the thermal comfort condition which was driven by the hot water temperature (Thot) of 90–98 °C. The system performance could vary with the heat source temperatures, cooling load, primary nozzle, and air-conditioned space temperature. The optimal performance was determined when varying the Thot, and, hence, the optimal Thot was indicated. The optimal Thot varied significantly with variations in the working condition. The test results demonstrated high potential to further using the ejector refrigeration system in the actual air conditioning application.

Comprehensive performance analysis and optimization of 1,3-dimethylimidazolylium dimethylphosphate-water binary mixture for a single effect absorption refrigeration system

The energy and exergy analyses of the absorption refrigeration system (ARS) using H2O-[mmim][DMP] mixture were investigated for a wide range of temperature. The equilibrium Duhring (P-T-XIL) and enthalpy (h-T-XIL) of mixture were assessed using the excess Gibbs free non-random two liquid (NRTL) model for a temperature range ??? of 20°C to 140°C and XIL from 0.1 to 0.9. The performance validation of the ARS cycle showed a better coefficient of performance (COP) of 0.834 for H2O-[mmim][DMP] in comparison to NH3-H2O, H2O-LiBr, H2O-[emim][DMP], and H2O-[emim][BF4]. Further, ARS performances with various operating temperatures of the absorber (Ta), condenser (Tc), generator (Tg), and evaporator (Te) were simulated and optimized for a maximum COP and exergetic COP (ECOP). The effects of Tg from 50°C to 150°C and Ta and Tc from 30°C to 50°C on COP and ECOP, the Xa, Xg, and circulation ratio (CR) of the ARS were evaluated and optimized for Te from 5°C to 15°C. The optimization revealed that Tg needed to achieve a maximum COP which was more than that for a maximum ECOP. Therefore, this investigation provides criteria to select low grade heat source temperature. Most of the series flow of the cases of cooling water from the condenser to the absorber was found to be better than the absorber to the condenser.

A Study on the Combined Driven Refrigeration Cycle Using Ejector

The rising need for thermal comfort has resulted in a rapid increase in refrigeration systems’ usage and, subsequently, the need for electricity for air-conditioning systems. The ejector system can be driven by a free or affordable low-temperature heat source such as waste heat as the primary source of energy instead of electricity. Heat-driven ejector refrigeration systems become a promising solution for reducing energy consumption to conventional compressor-based refrigeration technologies. An air-conditioning system that uses the ejector achieves better performance in terms of energy-saving. This paper presents a study on the combined driven refrigeration cycle based on ejectors to maximize cycle performance. The experimental setup is designed to determine the coefficient of performance (COP) with ejector nozzle sizes 1.8, 3.6, and 5.4[Formula: see text]mm, respectively. In this system, the R-134a refrigerant is considered as a working fluid. The results depict that the efficiency is higher than that of the conventional refrigeration method due to comparing the performance of the conventional refrigeration cycle and the combined driven refrigeration cycle. The modified cycle efficiency is better than the vapor compression cycle below 0∘C, which implies sustainability at low temperatures by using low-grade thermal energy. For the improvement of mechanical efficiency, proposed cycle can be easily used.

Temperature drop of heating fluid as a primary condition for effective utilization of low-grade heat using flash cycles and zeotropic mixtures in refrigeration ejector systems

ABSTRACT The conventional refrigeration ejector cycle utilizes a constant temperature process during motive heat consumption needed for vapor generation. This may be thought of as the main restriction concerning the application of low-grade heat as a motive source by these systems since it results in required significant mass flow rate with a small temperature drop of heating fluid. The first proposed novel approach is an ejector flash cycle (EFC) in which the working fluid is heated up to the saturation temperature only at the inlet to the primary nozzle of the ejector. The second possible approach is the application of a zeotropic mixture as the working fluid for the conventional ejector cycle. The performance of an EFC is compared to the ejector refrigeration cycle (ERC) working with zeotropic refrigerant. Three motive heat source temperatures (60°C, 75°C, 90°C) and two evaporation temperatures (0°C, 6°C) were tested. Eight pure low-GWP refrigerants and four low-GWP refrigerant blends were analyzed. It was demonstrated that the flash cycle can efficiently operate with low-temperature motive heat source (60°C) with refrigerants R1234yf and R1234ze(E). The highest coefficient of performance (COP) is attainable for propane with a motive source temperature of 60°C and R1234ze(Z) for the motive temperature of 75°C and 90°C. The differences of liquid pump power consumption of both discussed cycles compared to the conventional compression cycle are significant and reach up to 90 percent in the case of EFC and up to 96 percent in the case of the zeotropic cycle. The electric power savings in the case of the zeotropic cycle are slightly higher, but COP and drop of the heating water temperature are notably lower. Therefore, the EFC system is suggested to be a better solution for low-grade heat utilization in the cases when a significant decrease of the motive source fluid temperature is required.

Performance evaluation of a co-production system of solar thermal power generation and seawater desalination

Increasing power cycle efficiency is an important way to reduce the cost of the solar thermal power generation. The power generation system using a supercritical carbon dioxide (s-CO2) Brayton cycle has the advantages of high cycle efficiency, small equipment size and low corrosion. Then, low temperature waste heat generated by the s-CO2 Brayton cycle can be used as the low temperature heat source for multi-effect desalination. This paper discusses a s-CO2 Brayton cycle integrated with a multi-effect seawater desalination system. The heat input of the s-CO2 Brayton cycle is from a central-tower solar receiver. The residual heat of the s-CO2 Brayton cycle is the heat input of the multi-effect distillation system. The results show that the integration of multi-effect desalination system does not affect the s-CO2 Brayton cycle efficiency. With the increase of split ratio of the compressor, Brayton cycle efficiency increases and then decreases, and the water production decreases and then increases. With the increase of turbine inlet temperature, Brayton cycle efficiency increases while freshwater production varies insignificantly. After optimizing the design of the cogeneration system, solar thermal power efficiency is 24.04%, the cost of electricity generation can be reduced, the LCOE is 0.081$/kWh. by using the waste heat generated from the power cycle. realized electricity-water cogeneration, the fresh water production from 5-effect MED is 459m3/day, and the unit cost of freshwater UPC of 0.81$/m3.

Thermodynamic Efficiency Maximum of Simple Organic Rankine Cycles

The increase of the maximal cycle temperature is considered as one of the best tools to increase cycle efficiency for all thermodynamic cycles, including Organic Rankine Cycles (ORC). Technically, this can be done in various ways, but probably the best solution is the use of hybrid systems, i.e., using an added high-temperature heat source to the existing low-temperature heat source. Obviously, this kind of improvement has technical difficulties and added costs; therefore, the increase of efficiency by increasing the maximal temperature sometimes has technical and/or financial limits. In this paper, we would like to show that for an ideal, simple-layout ORC system, a thermodynamic efficiency-maximum can also exist. It means that for several working fluids, the thermodynamic efficiency vs. maximal cycle temperature function has a maximum, located in the sub-critical temperature range. A proof will be given by comparing ORC efficiencies with TFC (Trilateral Flash Cycle) efficiencies; for wet working fluids, further theoretical evidence can be given. The group of working fluids with this kind of maximum will be defined. Generalization for normal (steam) Rankine cycles and CO2 subcritical Rankine cycles will also be shown. Based on these results, one can conclude that the increase of the maximal cycle temperature is not always a useful tool for efficiency-increase; this result can be especially important for hybrid systems.

Thermo-chemical engines: Unexploited high-potential energy converters

Thermal engines, particularly closed power cycles, are currently a focus of many studies mainly because they represent the only way to exploit renewable thermal energy. To increase the exploitation of available thermal sources, this work investigates the higher potential offered by a complementary technology based on the use of reactive working fluids instead of inert fluids: the here-called “thermo-chemical” engine. Such a power cycle enables the simultaneous conversion of thermal and chemical energy into work. Based on a theoretical approach, this paper explores engine performance considering different stoichiometries and thermodynamic characteristics of reactive fluids and different operating conditions. It is shown that the use of specific equilibrated reactions occurring in the gaseous phase might lead to extremely powerful and highly efficient energy conversion systems in the whole current domain of the application of power cycles. Moreover, it is demonstrated that, unlike classical thermal machines, a thermo-chemical engine allows efficient and powerful exploitation of low-temperature heat sources and high-temperature cold sinks, which in general, characterize renewable thermal energy.

Innovative technologies for energy production from low temperature heat sources: critical literature review and thermodynamic analysis

Low-temperature heat sources could be exploited by innovative technologies for producing work. The challenge is improving the efficiency and power density.

Effect of adsorption kinetics on the performance of the vapor compression-adsorption hybrid cycle

The adsorption cooling system is considered as environmental-friendly technology that can be driven by low temperature heat source. We focus on adsorption-vapor compression hybrid refrigeration system which is one of the reasonable options to reduce the consumption of fossil fuels. In this system, the evaporator of adsorption cycle work as the evaporative subcooler for the refrigerant of vapor compression cycle. Performance predictions of the hybrid system require to solve unsteady state governing equations of adsorption cycle including the adsorption characteristics parameters. In our study, the performance of the hybrid system was analyzed by calculating the adsorption kinetics in the model. We have calculated in many conditions such as different adsorption and desorption time or heat medium mass flow rate ratio. The result showed that unsteady model could grasp the adsorption characteristics on system performance analysis. This system has a capacity to improve the vapor compression cycle performance as much as 10% by using 1kW waste heat depend on operating conditions.

Thermodynamic analysis of a Carbon Carrier Cycle (CCC) for low temperature heat recovery

The aim of the paper is to study the thermodynamic behavior of a non-conventional power cycle, named Carbon Carrier Cycle (CCC), which is expected to obtain interesting performance with low temperature heat source. The CCC may be regarded as derived from an absorption machine, where an expander replaces the condenser, the throttling valve and the evaporator. The working fluid is a mixture of CO2 and a proper absorber. In the paper, the thermodynamic model of this kind of cycles is described, and the results obtained considering Acetone as the absorber are discussed. A first performance comparison is then conducted with a more conventional Organic Rankine Cycle (ORC).

Study of the integration of a supersonic impulse turbine in a NH3/H2O absorption heat pump for combined cooling and power production from a low temperature heat source

The present work is focused on the investigation of an absorption cycle integrated with an impulse axial turbine for the combined production of cooling and electric power. This technology holds great promise for its ability to harness low-temperature heat sources, more effciently in comparison to separate production with simple cycles. By developing a 1D model of the expander, and integrating it into a 0D model of the complete cycle, it is possible to evaluate the performance of the cycle and its variation with respect to the operating parameters, namely the temperature of the external resources. Pending an experimental validation of the results, this study showed the importance of correctly defining the temperature of the sources - namely the generator temperature - in order to satisfy the technological needs while also maximising the effciency of the cycle. Finally it was highlighted how the integration of a supersonic impulse turbine strongly limits the flexibility during operation given the constant mass flow rate treated by the expander.

Exergy and energy analysis on coal/biomass co-gasification in supercritical water

Supercritical water gasification (SCWG) is a promising technology for clean and efficient utilization of carbonaceous organic materials at high temperature and pressure. Coal/biomass co-gasification in supercritical water (SCW) is a better choice for both coal and biomass to offset their disadvantages. Therefore, based on the experimental results of coal/carboxymethylcellulose (CMC, as a model compound of biomass) co-gasification in SCW by continuous flow thermal-catalytic reaction system at a reactor wall temperature of 650 �C, pressure of 25 MPa, a residence time 30 s and 0.1 wt% NaOH additive, the effects of heat transfer efficiency, heat supply methods, and CMC fraction on exergy and energy efficiency of reactor (the core device in reaction system) were investigated. The results show that energy and exergy efficiencies are in excess of 69% and 43%, respectively. The priority order of heat supply for the reactor is as follow: lower temperature heat source > higher temperature heat source > direct electricity heat supply method. The heat transfer efficiency has great influence on the energy and exergy efficiencies in terms of thermophysics. The higher CMC fraction is helpful to improve exergy efficiency.

Comparative analysis and evaluation of different working fluids for the organic rankine cycle performance

The aim of this study is to find the best working fluid for Organic Rankine Cycles (ORCs) at specific temperature limits, with a focus on thermodynamic analysis, environmental impact, and protection. A subcritical Rankine cycle with organic working fluids was used to produce electricity using a low-temperature heat source. The six organic fluids (R600, R245fa, R290, R134a, R123 & R600a) were selected, studied, and compared for ORC applications to get high efficiency and COP. The study found that because of pressure factor, efficiency, expander, R600a, R134a & R290 are the most preferable choices. In terms of COP and pressure ratio, R600a is best among the six fluids. However, if we consider factors like mass flow rate, refrigerating capacity and COP, then R245fa & R123 are the suitable choices. However, the ODP of R123 is found to be 0.012, and given the targeted zero tolerance for ODP compounds, R123 working fluid would be phased out sooner than expected, especially given its significant GWP of 76. The GWP of R245fa is at an intolerable level of 820.

Influence of the Thermal Energy Storage Strategy on the Performance of a Booster Heat Pump for Domestic Hot Water Production System Based on the Use of Low Temperature Heat Source

Energy recovery from a low temperature heat source using heat pump technology is becoming a popular application. The domestic hot water demand has the characteristic of being very irregular along the day, with periods in which the demand is very intensive and long periods in which it is quite small. In order to use heat pumps for this kind of applications efficiently, the proper sizing and design of the water storage tank is critical. In this work, the optimal sizing of the two possible tank alternatives, closed stratified tank and variable-water-volume tank, is presented, and their respective performance compared, for domestic hot water production based on low temperature energy recovery in two potential applications (grey water and ultra-low temperature district heating). The results show that the efficiency of these kind of systems is very high and that variable-water-volume tanks allow a better use of the energy source, with an 8% higher exergy efficiency and around 3% better seasonal performance factor (SPF), being able to provide similar comfort levels with a smaller system size.

Modelling of urban excess heat use in district heating systems

Use of low-temperature heat sources in district heating (DH) systems is gaining attention as a climate mitigation measure. Urban excess heat (UEH) from data centres, metro stations, sewage systems and service sector buildings’ cooling system belongs to these sources. The use of UEH affects energy supply systems and system costs. We analyse the long-term system impacts of utilizing the four UEH sources through dynamic modelling of the energy systems. The study draws on data from real cases in four cities and addresses local conditions. The results show that UEH utilization can contribute to increased competitiveness of DH compared to individual heating. The system benefits are larger if UEH replaces boilers in DH system and/or in buildings.

Використання теплових насосів для опалення та гарячого водопостачання будівель підприємств на прикладі установок Helioterm

Heat pump - a device for transferring thermal energy from a source of low potential thermal energy to a consumer with a higher temperature. The thermodynamic cycle of a heat pump is similar to a refrigerating machine. Depending on the principle of operation, heat pumps are divided into compression and absorption. The most commonly used compression heat pumps. In recent years, numerous publications on the use of heat pump technology in heating and hot water supply facilities of various spheres - from individual homes to residential neighborhoods have appeared in various media. The authors of the publication have many years of experience in joint scientific and technical cooperation with leading technical universities and industrial organizations in the field of development and practical use of heat pump technology. The authors analyze the possibilities of introducing heat pumps at enterprises and organizations of the State Reserve System of Ukraine. It has been shown that the amount of expenses in comparison with central heating or operation of gas and electric boilers of similar power is several times smaller. It is noted that the implementation of heat pumps is a promising direction in the use of alternative energy sources to meet the heating, ventilation and hot water supply needs of buildings. The payback period from the introduction of heat pumps at enterprises is 4-9 years, depending on the location of the object and the type of source of low-temperature heat. The article meets the requirements of the State Tax Code of Ukraine and can be recommended for publication.

Analysis of the performance of an electricity generation system using the CO2 hydrate formation and dissociation process for heat recover

In Japan, approximately 70% of the primary energy supply is converted into low-temperature (lower than 200 °C) waste heat, which represents a small energy density; therefore, its utilization is limited to applications such as heat pumps, organic Rankine cycle, etc. In this paper, therefore, work from the heat source with a small difference in temperature is obtained using a gas hydrate power cycle. Furthermore, a new electric generator system is proposed. In the gas hydrate power cycle, gas hydrate is produced due to a low-temperature heat source (environment of a cold area), and then dissociated, resulting in high-pressure gas, due to a high-temperature heat source. In this cycle, the temperatures of the heat sources were set at −5 °C and 15 °C; therefore, in the proposed system, cold environment and waste heat can be considered as the low and high temperature heat source, respectively. With this system, the dissociation efficiency of CO2 hydrate when using a generation catalyst was at 42.5%, and the instant power conversion percentage of the actuator and generator was at 40.2%. Finally, 17.1% of the heat provided by the high-temperature heat source was converted into electric power by CO2 hydrate power cycle.