h i g h l i g h t sThe paper presents a thermodynamic analysis of A-CAES using packed bed regenerators. The packed beds are used to store the compression heat. A numerical model is developed, validated and used to simulate system operation. The simulated efficiencies are between 70.5% and 71.1% for continuous operation. Heat build-up in the beds reduces continuous cycle efficiency slightly. a b s t r a c tThe majority of articles on Adiabatic Compressed Air Energy Storage (A-CAES) so far have focussed on the use of indirect-contact heat exchangers and a thermal fluid in which to store the compression heat. While packed beds have been suggested, a detailed analysis of A-CAES with packed beds is lacking in the available literature. This paper presents such an analysis. We develop a numerical model of an A-CAES system with packed beds and validate it against analytical solutions. Our results suggest that an efficiency in excess of 70% should be achievable, which is higher than many of the previous estimates for A-CAES systems using indirect-contact heat exchangers. We carry out an exergy analysis for a single charge-storage-discharge cycle to see where the main losses are likely to transpire and we find that the main losses occur in the compressors and expanders (accounting for nearly 20% of the work input) rather than in the packed beds. The system is then simulated for continuous cycling and it is found that the build-up of leftover heat from previous cycles in the packed beds results in higher steady state temperature profiles of the packed beds. This leads to a small reduction (<0.5%) in efficiency for continuous operation.

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Hydrogen storage in a depleted gas reservoir or in an aquifer offers the potential for the seasonal storage of inherently variable renewable energy, by the electrolysis of water during periods of excess energy production. Here we investigate whether such storage is technically feasible. We compared the respective capacities and deliverabilities of hydrogen to established natural gas in a seasonal storage facility, on the basis of an estimated total volumetric capacity of 48MMm 3 , delivery pressures between 5-10MPa and emptying period of 120 days for the Rough Gas Storage Facility (UK). For the modelled scenario, an average power in the order of 4-5 GW would be required during a six month injection cycle to fill the reservoir to capacity. The equivalent hydrogen facility could store and supply 42% of the energy capacity supplied by its natural gas counterpart, and for an emptying period of 120 days could deliver power at an average rate of approximately 100 GWh/day, or ca. 40% of the energy deliverability of natural gas. There appears to be no insurmountable technical barrier to the storage of hydrogen in a depleted gas reservoir. Hydrogen losses from dissolution and diffusion could be reduced to less than 0.1%. Losses from biological conversion of residual CO2 were limited even with calcium carbonate dissolution. However, the biological reduction of sulphur minerals to hydrogen sulphide remained a potential problem.

Users of biochar in the field require this product to reliably meet its declared specifications.For the first time, this work investigated, whether these specifications could be reproducibly obtained as a sole function of the thermal history of the biomass feedstock during slow pyrolysis, irrespective of the type and scale of the production unit. Using volatile matter content as a proxy for a wider set of biochar quality parameters, biochar from units at scales from grams to hundreds of kilograms, representing three main types of slow pyrolysis units (fixed bed, screw reactor and rotary kiln) were investigated. For the first time we showed that comparable biochar could be produced by these very different pyrolysis units, with good reproducibility within individual as well as among separate production runs.

Meeting inter-seasonal fluctuations in electricity production or demand in a system dominated by renewable energy requires the cheap, reliable and accessible storage of energy on a scale that is currently challenging to achieve. Commercially mature compressed air energy storage (CAES) could be applied to porous rocks in sedimentary basins worldwide where legacy data from hydrocarbon exploration are available, and where geographically close to renewable energy sources. Here we present a modeling approach to predict the potential for CAES in porous rocks. By combining these with an extensive geological database we provide a regional assessment of this potential for the UK.

The Chemical Engineering Plant Cost Index (CEPCI) is widely used for updating the capital costs of process engineering projects. Typically, forecasting it requires twenty or so parameters. As an alternative, we suggest a correlation for predicting the index as a function of readily available and forecast macroeconomic indicators: with k o the first year of the period under consideration, i k the interest rate on US bank prime loans in year k, and P oil the US domestic oil price in year n. Best fit was obtained when choosing distinct sets of values of the constants A, B and C for each of the three periods

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