The title and abstract are as follows:
Simulations of air-blown thermal storage in a rock bed
This paper presents computer simulations of air-blown thermal storage in a loosely packed bed of rocks. An important application is storage of solar thermal energy for power generation or process heating. A new formulation is developed for one-dimensional flow of air through the rock bed, including the variation of density with temperature. The model equations are solved numerically and results given for the effect of important parameters such as particle size, depth of bed and air flow-rate. It is shown to be useful for the rock bed to be charged with downwards airflow and discharged with upwards airflow. This schedule is always superior – sometimes significantly so – to a schedule in which the bed is charged and discharged in the same (upwards) direction.
I began to think about these simulations in mid-2011 when I realised that thermal storage in pebble beds would be a good fit with my evaporation engine. I designed and coded the simulation algorithm in early 2012. In one part of the paper I compare my simulations with those of Hänchen et al. (2011) whose work also includes experimental validations. Those comparisons show that results from my code match well with experimental results. That’s always pleasing!
I then go on to look at the importance of molecular diffusion inside individual pebbles. For pebbles up to about 50 mm in diameter, you might as well just assume the pebbles heat up or cool down uniformly. Full molecular diffusion gives slightly more accurate results, but the extra accuracy hardly justifies the extra computation that is required.
Another topic I studied is the sharpness of the front between hot and cold zones in the bed. The front becomes sharper as the particle diameter decreases.
Yet another topic is the efficacy of charging and discharging through bi-directional (2-way) and uni-directional (1-way) strategies. In the 2-way strategy, the bed is charged with downwards airflow and discharged with upwards airflow. In the 1-way strategy, the airflow is always upwards. Representative results can be viewed at www.sunoba.com.au (follow the link to “Thermal storage simulations” on the right-hand side). My conclusion here is that the 2-way strategy is always to be preferred, even if it requires extra plumbing and ducting of the pebble bed.
The last section of my paper looks at two sorts of losses. For large beds, thermal losses from insulated beds are generally very small, typically of the order of 1% of the total useful heat content over a 24 hour period. I also show that the parasitic loss associated with pumping air through the pebble bed is acceptable for the sort of practical application I have in mind.
Since writing the paper, I have used the simulation code to investigate how pebble bed storage could be used in conjunction with my evaporation engine. This is the abstract for the conference paper that resulted (Barton, 2012):
A simulation study is presented for air-blown thermal storage in a solar thermal power station powered by passive heat collection under a transparent insulated canopy. The principal objective of this study is to investigate the round-trip efficiency of thermal storage in a pebble bed. In the proposed system, heat energy is converted to power by a new heat engine based on evaporative cooling of hot air at reduced pressure.
The work examines the performance of the canopy/engine/storage system over representative days each month for a full year. The useful heat reclaimed from the storage system is typically about 95% of the useful heat input, less small additional losses at the walls and ducts of the storage system. Because the heat reclaimed has a smoother daily temperature distribution than the heat gathered by the canopy, there is another 5% penalty in conversion of heat into power. For the configuration used in this study, the power output using storage is 88% of what would be obtained without storage. This estimate includes modest losses due to pumping and heat transfer at walls and ducts. Coarse economic evaluations indicate that storage would reduce the Levelised Cost of Electricity by 27% and increase the Capacity Factor of the engine by 88%.
I’m enthusiastic about the prospects for pebble bed thermal storage. The pebble bed simulation code will be useful for various applications including dispatchable solar thermal power generation and provision of process heat in domestic and industrial applications. The next application to be studied will be to my BRRIMS (Brayton-cycle, Re-heated, Recuperated, Integrated, Modular, Storage-equipped) concept for solar thermal power generation.
N.G. Barton, “Passive Solar Power Generation with Air-blown Thermal Storage”, Solar2012, Australian Solar Council, Melbourne (2012).
M. Hänchen, S. Brückner, A. Steinfeld, “High-temperature thermal storage using a packed bed of rocks – heat transfer analysis and experimental validation”, Applied Thermal Engineering, 31 (2011) 1798-1806.