Low Temperature Heat Sources Research Articles

Liquid-like adsorbent assembled by CNTs: Serving as renewable CO2 capture materials for indoor air

In this study, a CO2 capture material in the form of liquid-like adsorbents (LLAs) is developed to overcome the limitations of conventional types of adsorbents. The increase in indoor activities necessitates the capture of CO2 in enclosed indoor spaces. Indoor spaces require safe and stable materials for CO2 capture because humans are present in these spaces. Solid adsorbents are mainly used because liquid absorbents are unsuitable owing to noise and scattering problems. In LLA, the liquid absorbent assembled by carbon nanotubes (CNTs) is solidified and prevented from flowing and scattering indoor. LLAs present to maintain 95% of initial capacity after recycling 20 times, and have characteristics that can be regenerated in a low temperature heat source (80 to 120 ℃) and moisture resistance. This work not only provides indoor useable CO2 capture materials, but also offers a new prospect in the field of adsorbents.

Steady state behavior of a booster heat pump for hot water supply in ultra-low temperature district heating network

Employing booster heat pumps (BHP) in district heating systems can provide a way to use low temperature heat sources for district heating as they allow locally increasing the water temperature on the user side where necessary. Thereby, it can provide decreased heat loss of a district heating system. This experimental study investigated steady state thermal behavior of a BHP for cases with different forward temperature and different flow rates on the user side. The unit used R134a as working fluid.The results revealed that the Coefficient of Performance (COP) of the BHP system has an optimal value at the studied range. At a forward temperature of 45 °C the heat pump reached maximum COP of 4.95, heating capacity of 15.36 kW, and exergy efficiency of 33.4 %, respectively. Lorenz COP and Lorenz efficiency is ranged from 13 to 27 and 0.16 to 0.26, respectively. Exergy destruction of each component as well as exergy efficiency of BHP used in the proposed system have also been analyzed, which showed that the compressor take the highest the exergy destruction among the components. The exergy efficiency ranged from 24.9% to 33.4% in the considered conditions. The share of exergy destruction of condenser and evaporator were influenced by the heat transfer condition on both sides, which reveals that the BHP can achieve potential performance improvement by better matching heat transfer profile of condenser and evaporator.

Research on influence of high and low temperature heat sources for heat transfer characteristics of pulsating heat pipe cold storage device

Research on influence of high and low temperature heat sources for heat transfer characteristics of pulsating heat pipe cold storage device

Parametric investigation of open-drive scroll expander for micro organic rankine cycle applications

Organic Rankine cycles (ORC) are used to produce power from low-temperature heat sources. In the low power output range (

Experimental and numerical analysis on low-temperature off-design organic Rankine cycle in perspective of mass conservation

Experimental and numerical studies on the low-temperature heat recovery organic Rankine cycle operating in off-design conditions are conducted for the in-depth understanding of the system's underlying mechanisms in terms of mass conservation. Experimental data sets were obtained from a 1 kW lab-scale organic Rankine cycle test bed using R245fa as the working fluid. The effects of several boundary conditions, including charged mass, are thoroughly examined under low-temperature heat source within the range of 65–95 °C. Numerical models of the heat exchangers are developed by applying the discretization method to predict the captured mass inside the phase-changing components and validated within 5% error range. By the integration of experimental and numerical methods, unprecedented and critical results covering the pressure formation process, mass distribution, and liquid receiver modeling are derived from the analysis which could not be discovered through previous approaches. The unconventional thermodynamic state of the working fluid inside the liquid receiver is revealed in detail and a passive design is suggested for the liquid receiver model. An improved solver architecture is proposed for the complete development of a fully deterministic off-design organic Rankine cycle simulation model, where the reality-based logics obtained from the key findings are projected into the novel model.

Influence of control strategy on seasonal coefficient of performance for a heat pump with low-temperature heat storage in the geographical conditions of Central Europe

This paper presents a computational parametric study on increasing the Seasonal Coefficient of Performance (SCOP) for residential heat pumps. The studied system consists of a heat pump, low-temperature heat storage, and a control unit. The heat pump enables selection of a low-temperature heat source between ambient air and water in a tank. Two variants of low-temperature heat storage are tested, particularly, insulated-water heat storage and water heat storage sunken in soil. The study is further complemented with a test of selected algorithms for heat pump control: equithermal regulation, a binary algorithm for temperature source selection, a predictive algorithm for the heat storage discharging, and an algorithm for deferred heat storage discharging. A computational model of the system is made using Python. The assessment of HP operation is made based on meteorological data from the years 2008–2019 recorded in the city of Brno, Czech Republic, Central Europe. The results obtained show that using the approaches tested has the potential for increasing the SCOP. This increase reaches as much as 5.19% and it requires only a simple software change in the heat pump control algorithm and connection to meteorological data prediction.

Analysis of the thermodynamic performance limits of the organic Rankine cycle in low and medium temperature heat source applications

Analysis of the thermodynamic performance limits of the organic Rankine cycle in low and medium temperature heat source applications

Optimisation of Operation of Adsorption Chiller with Desalination Function

The demand for electricity is growing rapidly along with economic development and increasing population. At present, its production is mainly based on non-renewable sources, which has negative impacts on the environment and contributes to global warming. A large proportion of the produced electricity is consumed by refrigeration equipment. Climate change and the progress of civilisation are additionally increasing the demand for cooling, with increasing electricity consumption as a consequence. One of the options for obtaining eco-friendly cooling is the use of adsorption chillers. These devices are powered by low-temperature heat and their operation only requires a small amount of electrical energy. The source of low-temperature heat can be, e.g., waste heat generated in many industrial processes. Its use allows one to increase energy efficiency and achieve additional financial benefits. However, adsorption chillers are characterised by low coefficients of performance. This paper presents possibilities to improve their performance. It also presents the results of tests carried out on a three-bed adsorption chiller with desalination function. The aim of the investigation was to determine the effect of the cycle time on the coefficient of performance (COP) and specific cooling power (SCP). The working pair was silica gel and water. The results confirmed the effect of the duration of adsorption and desorption on the COP and SCP of the adsorption chiller. Increasing the duration of the cycle led to an increase in the COP.

Performance and operation strategy optimization of a new dual-source building energy supply system with heat pumps and energy storage

The use of renewable energy is an important technical way to achieve building energy conservation and environmental protection. In this study, a new type of dual-source building energy supply system with heat pumps and energy storage, which can solve the problems of unstable operation and low reliability of a single-energy system and high investment and operation cost of existing multi-source energy systems, is proposed. In this system, photovoltaic–thermal water is used as the low-temperature heat source of the water source heat pump and the air source heat pump as the auxiliary energy supply device to realize cascade utilization of energy. “Peak load shifting” and minimization of building energy consumption and operation costs are realized by storing energy (cold and heat) at night when electricity prices are low. On the basis of the long-term monitoring data of the energy supply system for an office building, load-forecasting and system performance models are established, and the operation strategy in winter and summer is optimized. Then, the whole-year operation of the system is simulated and analyzed. Results show that the optimized system runs efficiently and can save approximately 10% of operating costs compared with the situation before optimization. The operating cost of the system is 55% of that of the conventional air source heat pump system, and its dynamic payback period is 3.66 years. The proposed system is a form of building energy supply system worthy of popularization and application in cold regions.

Performance optimization of an integrated adsorption-absorption cooling system driven by low-grade thermal energy

• Thermal characteristics of integrated absorption - adsorption systems were studied. • Optimal intermediate pressure and LiBr solution concentration were investigated. • Effect of heating and cooling flow arrangement on system performance was evaluated. • The integrated system produced a greater cooling capacity with a reliable high COP. • The AB-bottom-AD-top system performed better than the AD-bottom-AB-top system. The integrated adsorption-absorption system is a novel technology to convert low-temperature waste thermal energy into useful cooling, which substantially improves energy utilization efficiency and lowers environmental pollution. This novel system incorporates the adsorption cycle into the absorption cycle so that the generation/adsorption pressure can be adjusted to enhance the system performance at low-temperature heat sources. In this paper, the thermal characteristics of the integrated system are theoretically evaluated. The optimization of the key parameters (e.g. cycle time, switching time, intermediate pressure and solution concentration) are conducted to achieve the best system performance. Furthermore, different configurations of the integrated system and the effect of the heating and cooling flow arrangements on the system performance are also investigated. The results showed that the coefficient of performance of the proposed system was as high as 0.4 at a heat source temperature of 50 °C. As the heat source temperature increased from 50 to 85 °C, an optimal intermediate (generation/adsorption) pressure varied gently from 2.36 to 2.16 kPa while the optimal solution concentration increased from 52.4 to 65% to achieve the best system coefficient of performance. The results also showed that the maximum specific cooling power is 193.7 W/kg when the heating and cooling water flow arrangement are both in parallel. In contrast, the lowest specific cooling power is 157.9 W/kg for both heating and cooling water flow arrangements in series. When compared the two different configurations, the cooling capacity and coefficient of performance of the configuration with absorption-cycle as the bottom cycle and adsorption-cycle as the top cycle were about 5 and 15% higher, respectively than those of the configuration with adsorption-cycle as the bottom cycle and absorption-cycle as the top cycle.

An experimental study in full spectra of solar-driven magnesium nitrate hexahydrate/graphene composite phase change materials for solar thermal storage applications

Magnesium nitrate hexahydrate is a preferred material in building heat applications with a moderate temperature (89 °C), suitable latent heat (130~150 kJ·kg−1), and low cost (1000~2000 CNY/Ton). However, as the low heat source temperature in the engineering application (usually less than 150 °C), the temperature difference driving the phase transition of the phase change materials is small, resulting in the slower dynamic process of heat storage. Aiming at this problem, this paper proposes a new method to prepare solar-driven composite phase change materials by combining photo-thermal materials with salt hydrate phase change materials. Solar-driven composite phase change materials can absorb photons and convert them into heat, most of which is stored in the phase change materials as the form of latent heat. The energy driving phase transition of phase change materials came from the solar and the other heat source simultaneously. Herein, a novel solar-driven composite phase change material containing magnesium nitrate hexahydrate, carboxymethyl cellulose, and graphene is prepared successfully and its absorption capacity and photo-thermal storage performance are measured systematically in the full spectrum. Compared with pure magnesium nitrate hexahydrate, the absorption capacity of the composites with graphene can be increased to 79.12% from 46.54%. The photo-thermal conversion and storage efficiency of composite phase change materials as high as 69.73%, and for the pure magnesium nitrate hexahydrate, it is not enough to absorb much heat to make a phase transition due to low absorption capacity. Also, the thermal conductivity of composite phase change materials with 5 wt% of graphene was significantly enhanced by 191.18% (0.99 W∙m−1∙K−1), compared with 0.34 W∙m−1∙K−1 of one for composites without graphene. Although with graphene mass fractions increases, the latent heat slightly decreases, an acceptable latent heat value (122.8 kJ∙kg−1) is obtained. The prepared sample has excellent cycling performance, which is important in applications. Therefore, this novel solar-driven composite phase change material could be potential as a solar heat storage material in engineering applications due to its good photo-thermal conversion and storage performance.

Thermodynamic Performance of Adsorption Working Pairs for Low-Temperature Waste Heat Upgrading in Industrial Applications

The present work aims at the thermodynamic analysis of different working pairs in adsorption heat transformers (AdHT) for low-temperature waste heat upgrade in industrial processes. Two different AdHT configurations have been simulated, namely with and without heat recovery between the adsorbent beds. Ten working pairs, employing different adsorbent materials and four different refrigerants, have been compared at varying working boundary conditions. The effects of heat recovery and the presence of a temperature gradient for heat transfer between sinks/sources and the AdHT components have been analyzed. The achieved results demonstrate the possibility of increasing the overall performance when internal heat recovery is implemented. They also highlight the relevant role played by the existing temperature gradient between heat transfer fluids and components, that strongly affect the real operating cycle of the AdHT and thus its expected performance. Both extremely low, i.e., 40–50 °C, and low (i.e., 80 °C) waste heat source temperatures were investigated at variable ambient temperatures, evaluating the achievable COP and specific energy. The main results demonstrate that optimal performance can be achieved when 40–50 K of temperature difference between waste heat source and ambient temperature are guaranteed. Furthermore, composite sorbents demonstrated to be the most promising adsorbent materials for this application, given their high sorption capacity compared to pure adsorbents, which is reflected in much higher achievable specific energy.

Heat pump systems: A mini review

In this work, a mini review of heat pumps is presented. The work is intended to introduce a technology that can be used to income energy from the natural environment and thus reduce electricity consumption for heating and cooling. A heat pump is a mechanical device that transfers heat from one environmental compartment to another, typically against a temperature gradient (i.e. from cool to hot). In order to do this, an energy input is required: this may be mechanical, electrical or thermal energy. In most modern heat pumps, electrical energy powers a compressor, which drives a compression - expansion cycle of refrigerant fluid between two heat exchanges: a cold evaporator and a warm condenser. The efficiency or coefficient of performance (COP), of a heat pump is defined as the thermal output divided by the primary energy (electricity) input. The COP decreases as the temperature difference between the cool heat source and the warm heat sink increases. An efficient ground source heat pump (GSHP) may achieve a COP of around 4. Heat pumps are ideal for exploiting low-temperature environmental heat sources: the air, surface waters or the ground. They can deliver significant environmental (CO2) and cost savings.

Simulation of Zinc-diffused InAs cells for low temperature thermophotovoltaic systems

We simulated zinc-diffused InAs cells which used in the low-temperature thermophotovoltaic system under 600–1200 °C blackbody radiation. By analyzing some important parameters, such as surface recombination speed or thickness of different InAs cells, we achieve the optimal cell structure. The Zn diffusion process was experimented at Zn–In alloy sources at different temperatures for 2 h and a box-shaped Zn profile with a single hump was obtained. We also performed a simulation for optimized GaSb structure. Unlike InAs cells, the GaSb cells have been investigated for a long time under various temperatures, from which we have obtained a better structure and a maximal efficiency of 30%. However, the output power density of InAs cells is greater than that of GaSb cells when applied under low temperature heat source below 1200 °C. Its open circuit voltage and short circuit current density were measured as 0.1157 V and 3.786 A, and the output power density was 0.2222 W/cm2, which is better than 0.098 W/cm2 of GaSb cell under a blackbody temperature of 800 °C with anti-reflection coating and 13.7% grid area. The improved performance can be attributed to its lower bandgap and better quantum efficiency (QE) curves.

A power plant for integrated waste energy recovery from liquid air energy storage and liquefied natural gas

Liquefied natural gas (LNG) is regarded as one of the cleanest fossil fuel and has experienced significant developments in recent years. The liquefaction process of natural gas is energy-intensive, while the regasification of LNG gives out a huge amount of waste energy since plenty of high grade cold energy (−160 °C) from LNG is released to sea water directly in most cases, and also sometimes LNG is burned for regasification. On the other hand, liquid air energy storage (LAES) is an emerging energy storage technology for applications such as peak load shifting of power grids, which generates 30%−40% of compression heat (~200 °C). Such heat could lead to energy waste if not recovered and used. The recovery of the compression heat is technically feasible but requires additional capital investment, which may not always be economically attractive. Therefore, we propose a power plant for recovering the waste cryogenic energy from LNG regasification and compression heat from the LAES. The challenge for such a power plant is the wide working temperature range between the low-temperature exergy source (−160 °C) and heat source (~200 °C). Nitrogen and argon are proposed as the working fluids to address the challenge. Thermodynamic analyses are carried out and the results show that the power plant could achieve a thermal efficiency of 27% and 19% and an exergy efficiency of 40% and 28% for nitrogen and argon, respectively. Here, with the nitrogen as working fluid undergoes a complete Brayton Cycle, while the argon based power plant goes through a combined Brayton and Rankine Cycle. Besides, the economic analysis shows that the payback period of this proposed system is only 2.2 years, utilizing the excess heat from a 5 MW/40MWh LAES system. The findings suggest that the waste energy based power plant could be co-located with the LNG terminal and LAES plant, providing additional power output and reducing energy waste.

Experimental investigation of an organic Rankine cycle with liquid-flooded expansion and R1233zd(E) as working fluid

A new concept of liquid-flooded expansion has been proposed as a performance increasing modification of the basic ORC targeted at low-temperature heat sources. However, little research demonstrates the potential of this technology especially experimentally. In this paper, an experimental test facility based on a conventional recuperative ORC system was constructed with an independent liquid flooding loop that enables testing the influence of liquid flooding on a modified single-screw expander as well as on the cycle itself. Experiments were performed at various pressure ratios (3.3–4.1) over the expander and flooding ratios (0–0.3) with R1233zd(E) as the working fluid and a standard lubricant oil as the flooding medium. The data reduction and uncertainty analysis were also discussed in depth. In total, 142 steady-state points were obtained. Compared with the baseline organic Rankine cycle, the maximum improvement of the liquid-flooded expansion on the expander power output can be 9.1%, although at slightly worse expander inlet conditions. The maximum enhancement of the isothermal efficiency of the expander was 9.5%. Results also showed that the expander power output, the net power output and the thermal efficiency were enhanced with the increase of the flooding liquid amount. The potential of an organic Rankine cycle system with liquid-flooded expansion can be further examined if over-expansion losses can be reduced and larger amount of oil can be injected, i.e., with higher pressure ratios and higher flooding ratios. Overall, this study provides insights into performance improvement by means of modifying the cycle thermodynamics itself.

Numerical study on uniformity of temperature difference field in a spiral tube heat exchanger

• Two equations are developed to construct the temperature difference field. • Two methods is proposed to measure the uniformity of continuous temperature field. • The proposed methods can be applied for other real-world applications. • New methods can characterize the heat transfer efficiency. The characteristic of heat transfer of an organic working fluid driven by a built-in spiral low-temperature heat source is investigated numerically. Field synergy number is a function of the heat transfer factor j with the coefficient of correlation (r) all above 0.9. The temperature difference field is built based on the time space and position information about the objective. New metrics for quantifying the uniformity parameters of continuous temperature fields are developed by temperature difference field modeling. Those metrics are illustrated by using numerical modeling and present significant correlation between field synergy number or heat transfer factor. Furthermore, both the presented temperature field data and the other numerical temperature field is used to verify the effectiveness of this method. This method not only can significantly quantify variation on the uniformity of a temperature difference field under the same working conditions, but can also characterize the heat transfer efficiency at different times under the same working condition. This shows that the uniformity factor established in this study can reflect the heat transfer efficiency of heat exchanger as a whole, can be used as a new evaluation index of continuous field heat transfer efficiency, and can provide a new idea of the synthesis and optimization of heat exchanger.

Comparative analysis of Ormat and Kalina Cycle for the geothermal resource in Mabini, Batangas using thermoeconomic analysis

The Philippines is amongst the countries located in the ring of fire, thus makes it rich in geothermal energy. The Department of Energy is considering the low-temperature resource for power generation to address the continuous increase in electricity demand. The binary plants designed by Ormat technologies are currently in use for power production for the Philippine low-temperature heat source. Another technology used for the low-temperature heat source is the Kalina power cycle. In this paper, both cycles are evaluated using the parameters from the Mabini, Batangas geothermal resource, and principles of thermoeconomics are applied to determine the performance of the two cycles under Philippine conditions.

R1224yd(Z), R1233zd(E) and R1336mzz(Z) as replacements for R245fa: Experimental performance, interaction with lubricants and environmental impact

Achieving the 2050 EU climate targets - an economy with net-zero greenhouse gas emissions - requires a significant increase in the share of renewables in the current energy mix. The Organic Rankine Cycle (ORC) could play an important role in this transformation of today’s energy systems. The ORC is able to convert low-temperature heat sources into electrical power by utilizing refrigerants as working fluids. Many of these refrigerants have a significant global warming potential (GWP) and recent legislations therefore increasingly limit their use. In recent years a new generation of refrigerants has been developed. These refrigerants are characterized by low global warming potentials and are considered as replacements for the old high-GWP refrigerants.In this paper R1224yd(Z), R1233zd(E) and R1336mzz(Z) are investigated as possible replacements for the state of the art high-GWP refrigerant R245fa. These novel fluids have quite similar thermophysical properties and can thus potentially be used as drop-in replacements for R245fa. For the application as drop-in replacement and the correct interpretation of measurement data, the interaction of the refrigerant with the used lubricant is important. Therefore, the interaction of all four refrigerants with a typical lubricant oil is experimentally investigated first. A novel model based on Raoult’s law is introduced and validated with experimental data. The model allows the computation of the liquid and vapor phase composition of refrigerant/oil-mixtures using pressure and temperature measurements. In the next step, the performance of the four investigated refrigerants is evaluated experimentally in an ORC test rig. Therefore, the same experimental program was repeated for every refrigerant in order to ensure a fair comparison. To conclude the evaluation of the refrigerants, a Life Cycle Assessment (LCA) is conducted for a theoretical geothermal power plant operated with the four considered refrigerants.The experimental results show, that R1224yd(Z) as well as R1233zd(E) are very suitable low-GWP alternatives for R245fa and could be used as drop-in replacements. R1336mzz(Z) on the other hand shows significantly lower system efficiencies and power outputs compared to the other refrigerants. The highest power output and system efficiency is however still achieved by the state of the art refrigerant R245fa, but these quantities are only slightly lower for R1233zd(E) and R1224yd(Z) . The LCA based on the experimental results for a hypothetical large-scale geothermal ORC application reveals, that despite the slight performance decrease, the application of low-GWP fluids decreases the CO2-equivalent of the system significantly. The usage of R1233zd(E) could decrease the CO2-equivalent by 67% compared to R245fa.

The Effect of Heat Removal during Thermophilic Phase on Energetic Aspects of Biowaste Composting Process

Composting is the natural, exothermic process where the huge amount of heat that is created is an issue of organic matter decomposition. However, too high temperature can reduce the microbial activity during the thermophilic composting phase. The aim of this study was to analyze the effect of heat excess removal from composted materials on the process dynamic. The experiment was performed in two parallel bioreactors. One of them was equipped with a heat removal system from the bed of the composted material. Three experiments were carried out with mixtures of different proportions: biological waste, wheat straw, and spent coffee grounds. The content of each option was determined based on a previous study of substrates to maintain the C/N ratio for the right composting process, provide adequate porosity composted material, and enable a proper degree of aeration. The study showed the possibility of receiving part of the heat from the bed of composted material during the thermophilic phase of the process without harm both to the course of composting and the quality of the final product. This shows that at a real scale, it can be possible to recover an important amount of heat from composted materials as a low-temperature heat source.