Proof of concept car adsorption air conditioning system using a compact sorption reactor

A prototype compact sorption generator using an activated-carbon ammonia pair based on a plate heat exchanger concept has been designed and built at Warwick University. The novel generator has low thermal mass and good heat transfer. The heat exchanger uses Nickel brazed shims and spacers to create adsorbent layers only 4 mm thick between pairs of liquid flow channels of very low thermal mass. The prototype sorption generator manufactured has been evaluated under the EU car air conditioning testing conditions. While driven with waste heat from the engine coolant water (at 90°C), a pair of the current prototype generators (loaded with about 1 kg of a carbon in each of two beds) has produced an average cooling power of 1.6 kW with 2 kW peaks

Optimisation of a recirculating domestic hot water system to minimise wait time and heat loss

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 for a 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 relationship between flow and warm-up rate to optimise pump size and timing

Performance characterisation and design considerations for a domestic ammonia/salt resorption heat pump

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 °C delivery; 60 °C for hot water is possible but with lower COP. The best results for space heating are obtained with source temperatures above -5 °C

Experimentally measured thermal masses of adsorption heat exchangers

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

Anisotropic thermal conductivity and permeability of compacted expanded natural graphite

The anisotropic thermal conductivities and permeabilities are investigated for discs and plates of compacted expanded natural graphite. The measuring directions of heat conductivity and permeability are both parallel and perpendicular to the pressing direction of compacted samples. An unexpected phenomenon is found in that the thermal conductivity sometimes decreases as the density of the material increases, and this phenomenon only occurs for thermal conduction parallel to the compressing direction. The results also indicate that the direction perpendicular to the compression direction shows higher thermal conductive properties and permeability values. Both anisotropic thermal conductivities and permeabilities are strongly dependent on density. Analysis shows that as a type of porous material, the ENG yields layers under the effect of pressure, and their orientation influences the values of heat conductivity and permeability of the different samples

Wall contact thermal resistance and bulk thermal conductivity in activated carbon from adsorption generators

It is acknowledged that the commercial success of the adsorption heat pump or refrigeration systems depends crucially on heat and mass transfer in the sorbent bed. Good heat and mass transfer allows one to obtain greater specific heating or cooling power which reduces the capital costs. This study surveys the previously reported data on heat transfer in activated carbon beds, compares it with newly measured data, and draws conclusions on the way that the wall geometry, particle size distribution, and the bulk density affect the overall thermal performance. The thermal conductivity technique used in this study is a steady-state measurement in a heated plate device

Thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid

The thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid (ENG-TSA) were measured both parallel and perpendicular to the direction of compression used to produce the samples. Results showed that the thermal conductivity and permeability were highly anisotropic. The thermal conductivity perpendicular to the direction of compression was 50 times higher than that parallel to the direction of compression and the permeability was 200 times higher. The maximum thermal conductivity measured was 337 W m(-1)K(-1) at a bulk density of 831 kg m(-3). The permeability perpendicular to the direction of compression varied in the range of 10(-11) to 10(-16) m(2) as the density increased from 111 to 539 kg m(-3). The specific heat was measured, and the average value is 0.89 kJ kg(-1) K(-1) in the temperature range 30-150 degrees C. As a type of heat transfer matrix the thermal diffusivity was about five times higher than that of, for example, pure aluminium due to the combination of improved thermal conductivity with comparatively low density and reasonable specific heat. (C) 2011 Elsevier Ltd. All rights reserved