With the increasing global demand for sustainable energy solutions, absorption-based thermochemical storage systems have emerged as promising candidates for long-term energy storage due to their high energy density. This study investigates the enhancement of absorption performance in an aqueous sodium hydroxide (NaOH) absorbent-based system through the addition of two widely used surfactants: 1-Octanol and 2-ethyl-1-hexanol. Experimental evaluations have been carried out for surface tension assessment and performance impact using a spiral finned heat exchanger under varying surfactant concentrations (100–500 ppm). Key performance metrics, including discharge power, storage density, effectiveness, and exergy efficiency, have been analyzed along with the associated heat and mass transfer characteristics. With the addition of 2-ethyl-1-hexanol at 300 ppm showing superior performance, further non-dimensional investigation has been carried out for varied operating conditions. The critical parameters affecting the surfactant induced enhancements have been identified through the non-dimensional analysis concerning heat and mass transfer. Furthermore, a simulation analysis for mass transfer enhancement along the heat exchanger length further elucidated the solution's vapor absorption potential as a key factor. Heat and mass transfer coefficient enhancements of up to 27% and 26%, respectively, have been observed for the solution, along with a significant potential of heat exchanger length reduction. Along with the length, the significance of fin width reduction in promoting flooding conditions has been ascertained through a comparative analysis with a wider finned heat exchanger. The findings underscore the potential of surfactants and compact heat exchanger designs in enhancing the efficiency and compactness of thermochemical energy storage systems.

Large temperature 'jump' and 'ramp' techniques, along with thermogravimetric analysis (TGA), were employed to characterise the sorption reactions of halide salt mixtures with ammonia. Using LTJ and FTR methods, a 1:1 mixture containing 50 % BaCl2 and 50 % BaBr2 by molar weight was studied but remained inconclusively characterised. To investigate further, samples with different BaCl2 and BaBr2 molar ratios (1:3, 1:1, and 3:1) were prepared. TGA results confirmed the presence of a new compound with sorption properties that vary in proportion to the molar ratios of the component salts. The effect of altering these molar ratios was evident: BaCl2-rich mixtures resembled pure BaCl2 salt behaviour, while BaBr2-rich mixtures behaved more like pure BaBr2 salt. Adsorption and desorption findings indicated an optimal bromide content at 50 %, beyond which the position of the reaction band becomes relatively stable-a pattern not observed in BaCl2-heavy mixtures. These distinct reaction "bands" expand the temperature operating range of sorption technologies by up to 30 degrees C, suggesting that tailored anion halide salt mixtures could be developed for advanced heat pumping and thermal storage applications.

This work has experimentally evaluated the position of thermochemical equilibrium lines of barium bromide (BaBr2) reacting with ammonia (NH3). BaBr2 samples, contained in discs of Expanded Natural Graphite, were placed in an experimental Large Temperature Jump rig and a new methodology, termed the Fast Temperature Ramp, was used to reveal adsorption and desorption reactions. Reaction onset points were plotted to generate equilibrium lines and calculate reaction enthalpies and entropies. Significant hysteresis between adsorption and desorption reactions was found, with the degree of hysteresis increasing as the salt becomes more deammoniated. The hysteresis effect was greatest for BaBr2(2-1) and BaBr2(1-0) reactions and also increased at higher pressures, exceeding 20 degrees C at 900 kPa for BaBr2(1-0). Adsorption reactions were found to occur over a very small temperature range, giving rise to a single transition 'zone' less than 20 degrees C wide. TGA experiments confirmed the position of equilibrium lines, although they were not successful in differentiating between two of the four ammoniated states. The close position of all equilibrium lines gives rise to a temperature 'zone' encompassing all four reactions, and in future work it may be advantageous to consider BaBr2 as a pseudo single-transition salt instead.

This paper introduces a novel binary salt mixture for use in thermal transformation, heat pumping, and thermal energy storage cycles. Samples were manufactured using the wet impregnation technique, with the large temperature jump (LTJ) technique used to analyse MnBr2 and MnCl2 reactions with ammonia, revealing equilibrium lines with hysteresis. Thermogravimetric analysis (TGA) demonstrates that binary salt mixtures of MnBr2 and MnCl2 exhibit sorption characteristics distinct from their individual salts. Varying the molar ratios in the mixtures shows that MnCl2-rich mixtures behave like pure MnCl2, while MnBr2-dominant mixtures resemble pure MnBr2. The unique behaviour of these mixtures, with reaction profiles spanning a wide temperature range, suggests the formation of a new molecular structure. This broadens the operational temperature range of sorption technologies by up to 30 degrees C, emphasising the potential of engineered anion halide salt mixtures in heat pumping and thermal energy storage applications.

Heat pumps will play a key role in the future provision of low carbon domestic heating and the re-use of industrial waste heat. Adsorption cycle heat pumps are advantageous in that they can use the existing natural gas network to avoid electricity supply limitations across the UK. A 2 kW domestic-scale ammonia/salt heat pump demonstrator is currently being tested at the University of Warwick as a replacement for a conventional condensing gas boiler. This paper describes analysis work in support of this testing which will lead to design refinements in follow-on developments. A Matlab-based 2D transient simulation package was developed to study heat transfer and reaction rate within a pair of linked reactors. Heat conduction and sorption rate are modelled together with inter-reactor gas flows and parasitic heat loss. Novel features include the use of Matlab's linked ODE solvers for convergence (ODE15S was found to be fastest) and the script file input configuration which combines clear visibility of parameters with the ability to run multiple simulations to show the effect of parametric variations. The code facilitates rapid design optimisation. Eleven cycle parameters have been investigated, including filling pressure, heat transfer coefficients, salt ratio, source temperatures, void space and heat capacity. The choice of cycle period involves a compromise between coefficient of performance and power output. A water/glycol heat transfer fluid gives better COP and output power than thermal oil. Insulation within the reactor shell has the potential to limit shell transient heat exchange but void space effects are likely to be more significant. The heat capacity of fluid in pipes and manifolds should be minimised. COP = 1.31 is achieved at 45 degrees C delivery; 60 degrees C for hot water is possible but with lower COP. The best results for space heating are obtained with source temperatures above-5 degrees C.

Using the Large Temperature Jump (LTJ) experimental technique, alongside a review of the literature, sodium bromide (NaBr) and manganese chloride (MnCl2) have been identified as a suitable working pair with ammonia refrigerant for a proof-of-concept resorption heat pump system. LTJ tests using a tube-side and shell-side unit cell reactor (sorption heat exchanger), show that the experimentally obtained equilibrium lines for adsorption and desorption of sodium bromide are: AHADS = 30,102.5 J/mol; ASADS = 207.7 J/(mol center dot K); AHDES = 30,216.4 J/mol; and ASDES = 206.8 J/(mol center dot K). Using a semi-empirical model, the NaBr composite salt (salt impregnated in expanded natural graphite (ENG)) has been characterised for use as a low temperature salt in a resorption heat pump, with manganese chloride as the high-temperature salt. The model constants, A and n, for adsorption are 1 and 3, and for desorption are 5 and 4 respectively for NaBr. Manganese chloride data has been previously reported (Hinmers et al., 2022). With an appreciation of the reaction dynamics and behaviour of the NaBr and MnCl2 composite salts, a proof-of-concept resorption system has been designed and manufactured. The reactor design, alongside the overall experimental rig design (including data acquisition system) is reported. Initial filling and flushing tests show the success of the data acquisition and control system, and thus the overall suitability of the proof of-concept system for investigations into the coupled nature of ammonia salt reactions for a resorption heat pump application.

To reduce the load on electric grids with the increasing usage of heat pumps (HP), thermochemical storage (TCS) integration is explored in the present study for heat discharge during peak hours. The potential of a liquid absorption based TCS with aqueous sodium hydroxide solution is considered for a domestic heating application. Three configurations of heat pump integration with the thermal storage are investigated for a typical building in UK. The optimal assessment of the thermal storage capacity and the operating temperatures has been carried out through an estimation of the thermodynamic limits and the total costs incurred over the system's lifetime. It is observed that the cascaded configuration with the TCS discharging heat to the room and heat pump connected to the ambient, offers the highest cost savings potential of up to 16% and an annual electric power consumption reduction of over 40% in comparison to that without thermal storage.

Many houses suffer from a delay in hot water arrival, after opening the hot tap, due to the length of pipe work from the boiler or thermal store. Measurements in three properties showed delays of up to 46 s. A recirculation system could maintain the water in the pipe at a suitable temperature and fora 15 m pipe is expected to use 30 W less electrical power than a local thermal store at the delivery point. Time-steady eigenvector solutions to the linked temperature equations allow optimisation of the recirculation flow rate, pipe diameters and insulation thickness. The pumping power is <1 mW and a pumpless thermo-syphon system should be possible in some installations. Heat loss coefficients have been calculated for a pair of pipes in a common insulation sleeve to minimise the losses. A plug flow transient advection model for water and pipe temperature based on a sliding 1-D grid has been developed and validated for a single pipe, then extended to also model recirculating systems. Heat losses in a pumped system may be reduced using a timer so the system can cool overnight. The transient model predicts the relation-ship between flow and warm-up rate to optimise pump size and timing.(c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

This work takes an empirical and evidence-based approach in the development of a resorption thermal transformer. It presents the initial modelling conducted to understand key performance parameters (coefficient of performance and specific mean power) before discussing a preliminary design. Experimental results from large temperature jump and isosteric heating tests have identified the importance of heat transfer in ammonia-salt systems. Both the heat transfer resistance between the salt composite adsorbent and the tube side wall, and the heat transfer from the heat transfer fluid to the tube side wall are key to realising resorption systems. The successful performance of a laboratory-scale prototype will depend on the reduction in these heat transfer resistances, and improvements may be key in future prototype machines. A sorption reactor is sized and presented, which can be scaled for length depending on the desired power output. The reactor design presented was derived using data on reaction kinetics constants and heat of reaction for calcium chloride reacting with ammonia that were obtained experimentally. The data enabled accurate modelling to realise an optimised design of a reactor, focusing on key performance indicators such as the coefficient of performance (COP) and the system power density. This design presents a basis for a demonstrator that can be used to collect and publish dynamic data and to calculate a real COP for resorption system.

The decarbonisation of building heating in urban areas can be achieved by heat pumps connected to district heating networks. These could be ‘third-generation’ (85/75 °C), ‘fourth-generation’ (50/40 or 50/25 °C) or ‘fifth-generation’ (near ambient) water loops. Networks using thermochemical reactions should require smaller pipe diameters than water systems and be more economic. This work investigates thermochemical transmission systems based on liquid–gas absorption intended for application in urban district heating networks where the main heat source might be a MW scale heat pump. Previous studies of absorption for heat transmission have concentrated on long distance (e.g., 50 km) transmission of heat or cold utilizing waste heat from power stations or similar but these are not directly applicable to our application which has not been investigated before. Absorbent-refrigerant pairs are modelled using water, methanol and acetone as absorbates. Thermodynamic properties are obtained from the literature and modelling carried out using thermodynamic analysis very similar to that employed for absorption heat pumps or chillers. The pairs with the best performance (efficiency and power density) both for ambient loop (fifth-generation) and high temperature (fourth-generation) networks use water pairs. The next best pairs use methanol as a refrigerant. Methanol has the advantage of being usable at ambient temperatures below 0 °C. Of the water-based pairs, water–NaOH is good for ambient temperature loops, reducing pipe size by 75%. Specifically, in an ambient loop, heat losses are typically less than 5% and the heat transferred per volume of pumped fluid can be 30 times that of a pumped water network with 10 K temperature change. For high temperature networks the heat losses can reach 30% and the power density is 4 times that of water. The limitation with water–NaOH is the low evaporating temperature when ambient air is the heat source. Other water pairs perform better but use lithium compounds which are prohibitively expensive. For high temperature networks, a few water- and methanol-based pairs may be used, but their performance is lower and may be unattractive.

This work has focussed on the development of an accurate method for testing and modelling the reaction kinetics involved in ammonia-salt adsorption reactions, something not achieved consistently to date. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. This was used to validate a new approach to modelling the behaviour and simulate the performance of chemisorption machines. A derivation of the heat transfer and thermodynamic equations are presented, and a finite difference model described which has been validated for the adsorption and desorption reactions of ammonia into halide salts within a porous matrix. The model is implemented in a MATLAB® program. Large Temperature Jump (LTJ) tests have been conducted on manganese chloride and barium chloride to validate the model and to identify the physical parameters which characterise the dynamic performance of the sorbent. The manganese chloride and barium chloride were impregnated in expanded natural graphite (ENG) (SGL SIGRATHERM®) board. The ENG board gave rise to practicable samples (31.5 mm OD ø over ½” tube) undergoing a desorption reaction in under 250 seconds with the fluid temperature 15°C above the equilibrium temperature, an order of magnitude faster than observed elsewhere. A new test method has been developed enabling an accurate single heat of reaction to be identified due to reduced hysteresis, which is reported for barium and manganese chloride. The model has been validated using experimental data from LTJ tests of two geometric configurations in radial heat transfer: discs heated/cooled from the outside radius (‘tube-side’) and annuli heated from the inner radius (‘shell-side’). The empirical data obtained is a milestone towards designed and optimised salt generators.

Gas-fired heat pumps are a potential replacement for condensing boilers, utilizing fossil-fuel resources more efficiently and reducing the amount of biogas or hydrogen required in sustainable gas grids. However, their adoption has been limited due to their large size and high capital cost, resulting in long payback times. For adsorption-based heat pumps, the major development challenge is to maximize the rate of heat transfer to the adsorbent, whilst minimizing the thermal mass. This work develops a modular finned-tube carbon–ammonia adsorption generator that incorporates the adsorbent in highly compacted 3-mm layers between aluminum fins. Manufacturing techniques that are amenable to low cost and high-volume production were developed. The module was tested using the large temperature jump (LTJ) method and achieved a time constant for adsorption and desorption of 50 s. The computational model predicted that if incorporated into two adsorption generators of 6 L volume each, they could be used to construct a gas-fired heat pump with a 10 kW heat output and a gas utilization efficiency (GUE, the ratio of useful heat output to higher calorific value of gas used) of 1.2.

In a resorption heat pump, the adsorption and desorption reaction of ammonium chloride (NH4Cl) with ammonia (NH3) is of interest as a Low Temperature Salt (LTS). Reviewing previously published NH4Cl-NH3 equilibrium lines, ammonium chloride appears to offer useable working temperatures (50–70 °C) in the 10–15 bar pressure range during the adsorption reaction, and provides beneficial working conditions for the desorption reaction, when compared with alternative LTS candidates at atmospheric pressure. The NH4Cl-NH3 adsorption and desorption reactions, using a NH4Cl composite salt, have been evaluated under dynamic ‘real-world’ conditions in a Large Temperature Jump (LTJ) experimental testing rig; although there are concerns with mass transfer characteristics, the salt exhibits no hysteresis between the adsorption and desorption reactions, contrary to previous literature. The experimentally obtained equilibrium line values for the reaction enthalpy and entropy are 29,835 J/mol and 207 J/(mol∙K), respectively. Using a semi-empirical model, the NH4Cl composite salt has been successfully characterised, enabling the prediction of salt reaction behaviour. The model constants, A and n, identified are 4.5 and 5 for adsorption and 5 and 4 for desorption, with an overall salt active fraction (applicable to both reactions) of 0.98. Overall, the working equilibrium line and the dynamic performance of ammonium chloride has been investigated and the applicability of NH4Cl as a LTS for a resorption heat pump determined.

The thermal masses of components influence the performance of many adsorption heat pump systems. However, typically when experimental adsorption systems are reported, data on thermal mass are missing or incomplete. This work provides original measurements of the thermal masses for experimental sorption heat exchanger hardware. Much of this hardware was previously reported in the literature, but without detailed thermal mass data. The data reported in this work are the first values reported in the literature to thoroughly account for all thermal masses, including heat transfer fluid. The impact of thermal mass on system performance is also discussed, with detailed calculation left for future work. The degree to which heat transfer fluid contributes to overall effective thermal mass is also discussed, with detailed calculation left for future work. This work provides a framework for future reporting of experimental thermal masses. The utilization of this framework will enrich the data available for model validation and provide a more thorough accounting of adsorption heat pumps.

The coupling of reversible ammoniation reactions between two salts presents a method for the exploitation of low grade waste heat. This resorption configuration can be used for thermal transformation or heat pumping, to recover waste heat to primary producers, or for integration in heat networks. To understand the solid/gas reaction behaviour and to model its kinetics, Large Temperature Jump (LTJ) experiments were performed on a composite of barium chloride in an expanded natural graphite (ENG) matrix. A model has been built using a semi-empirical equation from the literature, which has been validated with the LTJ results. The results suggest the semi-empirical model provides a reasonable prediction for solid/gas reactions once the constants have been identified. Enhancing the model to handle sequential phase change reactions will enable a wide number of salts to be modelled, making the design of a resorption system practicable.

The overall objective of the research is to develop a domestic air source gas-fired heat pump to replace standard condensing boilers cost-effectively. The working pair is ammonia and active carbon. A succession of sorption generators have been prototyped, culminating in the 'kebab' design in which monolithic carbon is compressed between aluminum fins on a steel tube containing pressurized water that either heats or cools the adsorbent in a two-bed cycle with heat and mass recovery. A 10 kW system was built and tested. Initial results were poor with Coefficient of Performance (COP) a little above 1. This was due to mass transfer resistance in adsorption being too high, due to an excessive binder fraction. A new formulation has been tested in a single 'kebab' and the results are consistent with an internal COP of 1.40 when delivering 10 kW at 50 degrees C and evaporating at 0 degrees C or 1.34 when delivering 20 kW.

There have been many attempts to commercialise and introduce heat-driven (particularly gas-fired) heat pumps over three decades. There are now two domestic systems on the market from Robur with others under development. The different types are reviewed, the markets assessed, and the barriers to wider uptake are discussed. Other options for future heating systems proposed within the UK are a range of electric heat pump developments and fuel cell/micro CHP units. Moving to an all-electric decarbonised electricity grid is shown to require a vast investment to perhaps triple the capacity of the electricity infrastructure and whilst possible in the long term cannot secure the emission reductions essential in the medium term. The case is proposed for a mixed heating solution with both gas-fired and electric heat pumps, also hybrids being used well into the 2040s. New-built houses will be almost exclusively electric and will need integration with advanced storage to supply domestic hot water. Older properties with higher heat loads will either use hybrid electric heat pumps, gas boiler systems, or gas-fired heat pumps. The proposed mix, whilst not being the minimal emission route, is much more affordable and a pragmatic solution to domestic heating.In addition, the research on a low-cost compact carbon-ammonia adsorption heat pump being carried out at the University of Warwick is described and the latest results discussed.

Current development of ammonia-carbon gas fired heat pumps at the University of Warwick uses shell and tube adsorption generators with over 1700 water tubes of 1.2 mm outer diameter on a 3 mm pitch filled with vibrated carbon grains and powder. This geometry is not optimised and a dynamic simulation program has been written to determine how far from optimal the design is and also whether an alternative design of finned tubes offer advantages.Three alternative carbon composites that use Expanded Natural Graphite (ENG), silane and lignin binders have been developed and tested to characterise their thermophysical properties so that they can be included in the simulations in order to improve the thermal transfer in the generators.Results presented show that the shell and tube geometry is close to optimal and that the best performing material is the lignin+Fcarbon composite.Other type of geometry, a finned tube design, was modelled as it might offer improvements in performance and help reduce the complexity and cost of the manufacturing technique. Results show that for the same tube radius, the finned tube generator needs 7 times fewer tubes in order to achieve similar performances. Crown Copyright (C) 2017 Published by Elsevier Ltd.

Thermochemical reactions, such as calcium chloride reacting with ammonia to form calcium chloride complexes, are attractive for application in heat pumps as they produce more heat per kg adsorbed sorbate but also adsorb a considerable higher amount of sorbate per kg of sorbent, compared to adsorbents such as zeolites. These benefits, however, come with together with a number of challenges. Firstly, the salts have poor thermal conductivity so the path from heat exchanger to the sorbent must be limited in order to allow for short cycling times and therefore high power density. Secondly, the salts typically swell and shrink upon (de)sorption, thereby easily losing their contact with the heat exchanger. To overcome these two problems, the salts are often placed in a matrix, such as expanded natural graphite (ENG) or zeolites. This paper shows the performance of a reactor containing approximately 1 kg of CaCl2 placed in a 1 kg ENG matrix. Its performance in terms of adsorption/desorption rates, heat input and output as a function of temperature and pressure gradients and under typical heat pump and transformer conditions is shown and compared with model calculations. The parameters used in the model calculations have been obtained from literature or independently measured using apparatus such as Rubotherm microbalance and a large temperature jump setup. The results show that material properties measured on small samples do not easily yield a proper description of the sorbent performance on kW-scale reactors. Some suggestions are made to improve future model description and experiments. (C) 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

This paper presents the development and thermal properties study of three types of carbon composites with enhanced thermal properties along with the optimisation of the geometry of the shell and tube generators currently used at the University of Warwick for the development of carbon-ammonia heat pumps. The three carbon composites developed use lignin binder, silanes binder and Expanded Natural Graphite (ENG) as a way of enhancing their thermal properties in order to perform more efficiently in a refrigeration or heating system. Two techniques were used in order to obtain the thermal properties: Anter and HyperFlash thermal conductivity instruments. Results show that ENG increases drastically the thermal conductivity of the samples (up to 3.4 W/(mK)) but reduces the carbon density (450 kg/m(3)) and increases the thermal mass of the system. Lignin binder samples show lower thermal conductivities (0.3 W/(mK)) but higher carbon densities (750 kg/m(3)d) which increases the performance of the machine. Thermal conductivities of samples were also obtained for a wide range of working temperatures (25-200 degrees C). The current development of gas fired heat pumps at Warwick uses shell and tube adsorption generators with over 1000 water tubes of 1.2 mm diameter on a 3 mm pitch. This geometry is not optimised and a dynamic simulation program has been written to determine how far from optimal the design is and also whether alternative designs offer advantages. The results presented show that the shell and tube is close to optimal for its type but that finned tube designs might offer improvements. (C) 2017 Elsevier Ltd. All rights reserved.