District heating of buildings by renewable energy using thermochemical heat transmission

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

Modelling and development of a generator for a domestic gas-fired carbon-ammonia adsorption heat pump

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+carbon 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

Alternative monolithic/composite carbons for adsorption generators and simulation for optimal performance

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/m3) 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/m3) which increases the performance of the machine. Thermal conductivities of samples were also obtained for a wide range of working temperatures (25–200 °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

Study of thermal conductivity and geometry wall contact resistance effect of granular active carbon for refrigeration and heat pumping systems

The commercial success of sorption refrigeration and heat pump systems depends on a good heat and mass transfer in the adsorbent bed, which allows higher coefficients of performance and greater specific heating or cooling power that reduce capital costs. In this study the thermal conductivity and thermal contact resistance of vibrated and compressed granular active carbon and binary mixtures of active carbon are investigated using two types of conductivity measurements: a steady-state measurement between flat plates and a transient hot tube measurement. With these results is possible to draw conclusions on how the wall geometry, particle size distribution, and bulk density affect the overall thermal performance. Results show that using binary mixtures of grains and powder gives results superior to those of either grains or powder alone. The conductivity of the binary mixtures increases roughly linearly with bulk density and the 2/3 grain mixture achieves the highest densities. The method used to achieve compaction (vibration or compression) did not seem to affect the result. Thermal contact resistances reduce with increasing density but do vary with the mixture ratio, also appearing to be best with a 2/3 grain–1/3 powder mixture

Thermodynamic and heat transfer analysis of a carbon – ammonia adsorption heat pump

The modelling, design, construction and experimental testing of a carbon-ammonia adsorption heat pump is presented. The main objective of the research was to computer simulate and test a 4-beds thermal wave adsorption cycle and to improve the heat transfer rate in an existing shell and tube generator. The existing generators were shell and tube type and were made of nickel brazed stainless steel but their heat transfer performance was poor. New heat exchangers with same design but larger in size were manufactured. The sorbent material, active carbon, was tested in order to characterise its thermal properties and a new generator filling technique was developed and presented. Computational modelling was carried out to evaluate the performance of the 4-beds thermal wave adsorption cycle. The proposed system was an air source heat pump that could deliver an output heating power of 7 kW and a seasonal heating COP of 1.47. The adsorption generators were tested in a 4-bed thermal wave air-source heat pump system and achieved heating output powers between 4.5 to 5.20 kW if taking into account the system heat losses (4.30 to 4.90 kW without heat losses) and heating COP’s of between 1.26 and 1.31 if taking into account the system heat losses (1.13 to 1.18 kW without heat losses). These values were significantly lower than the predicted performance of the simulation. The main cause of this discrepancy was the water distributors located at the end of the generators that distorted during the testing stage and blocked the tubes of the generators

Ammonia-carbon adsorption cycle refrigerators, heat pumps and thermal transformers

Active carbon-ammonia cycles have been developed from the 1980’s in the USA and are still a major research interest at the University of Warwick, where systems have been built for car air conditioning, solar refrigeration and gas-fired heat pumps. The basic cycles are introduced and a brief description of the historical development is presented. The paper then presents past projects on air conditioning and cooling and work to date on domestic gas fired heat pumps with preliminary results from the latest system under test, and describes plans and the prospects for future products. The possibility of using carbon-ammonia in thermal transformers for industrial use is modelled theoretically and the prospects discussed and compared with using ammonia and chemical salts