Two months ago, I made a blog post about computer simulation of air-blown thermal storage in a pebble bed. I’ve made further progress with this work, as I’ll now describe. I think there is one particularly interesting result.
To summarise the state of play two months ago …
For some years now, I have been working on a concept for passive solar thermal power generation. This involves a thermodynamic cycle based on evaporative cooling of hot air at reduced pressure. The energy to power the engine is provided by sunshine and collected passively under a transparent insulated canopy.
Details can be found at www.sunoba.com.au. According to my estimates, the Levelised Cost of Electricity would be highly competitive with other forms of solar power.
The possibility of cheap thermal storage would greatly enhance the concept. To accomplish this, simply blow hot air from the canopy through a pebble bed during the day. At night, cool air is drawn through the bed by the engine, heating up in the process, with the reclaimed thermal energy converted into electrical power.
Pebble beds offer a cheap thermal storage medium with unlimited cycles. Moreover, air is the heat transfer fluid and the working gas of the evaporation engine, so heat exchangers and condensers are not needed. That helps to make the canopy-storage-engine concept competitive, even if the operating temperatures are not particularly high.
Two months ago, I’d developed suitable model equations and successfully developed computer code to simulate the charge/discharge process. In these simulations, the airflow was always in an upwards direction (i.e. for both charging and discharging).
In the original model, air was an ideal gas with variable density but constant pressure and constant specific heat capacities. I allowed heat to diffuse radially in the rock particles. I included diffusion in the air but no inter-particle diffusivity. I solved my equations by a finite volume procedure.
What has happened in the past two months?
Firstly, I explored the importance of radial heat diffusion in the individual rock particles, assumed to be spheres. I introduced an option in which the temperature in the individual rock particles was uniform and governed by heat transfer between air and rock. Whilst there is a difference between the results for instant radial heat transport and molecular radial diffusion, the difference is small even for large particles (50-100 mm diameter). For rock particles around 20 mm diameter, the instantaneous diffusion model gives perfectly acceptable results and saves considerably on computer time.
Secondly, I introduced an alternative charge/discharge strategy. To be precise, the new “2-way” strategy involves charging by downwards flow of hot air through the bed and discharging by upwards flow of cold air. That is to be distinguished from the original “1-way” strategy in which the direction of air flow through the bed was upwards for both charging and discharging.
Thirdly, I explored the sensitivity of the baseline case to variations in depth of bed, mass flow-rate of air and particle diameter.
I’m currently writing up these results for publication, prior to embarking on another round of knocking on doors looking for investors.
Let me give a taste of the results.
The 2-way strategy is always superior to the 1-way strategy, sometimes dramatically so. For best reclaim of stored heat, use the 2-way charge/discharge strategy with a deep bed comprised of small (10-20 mm diameter) particles. The mass flow-rate of air should be modest. Under these circumstances for my canopy-storage-energy system, nearly 95% of the heat provided at the inlet to the pebble bed can be reclaimed.
The figures below show results for a 3.5 m bed of 20 mm steatite particles, with air mass flow-rate of 0.225 kg/(m^2.s). The bed is charged for 6 hours at charging temperature 140°C and then discharged for 6 hours with inlet temperature 20°C. The bed is assumed to have sufficiently large diameter that thermal losses on the side walls can be neglected. Results are shown for day 5 of the simulations for both 1-way and 2-way strategies.
For these results, the amount of energy reclaimed during discharge on day 5 is 75% for the 1-way strategy and 93% for the 2-way strategy. I think the effectiveness of the 2-way strategy is particularly interesting.
Please get in touch if you’d like a conversation about air-blown thermal storage, for whatever purpose.
Fig 1: Simulation of air-blown thermal storage in a bed of steatite rocks; 1-way charge/discharge strategy, results for day 5. Inlet temperatures 140°C (charge, 0 to 6 hrs) and 20°C (discharge, 6-12 hrs), initial bed temperature 20°C, air mass flow-rate 0.225 kg/(m^2.s), particle diameter 0.020 m, bed height 3.5 m. The air temperature is shown as a function of depth at each hour.
Fig 2: Simulation of air-blown thermal storage in a bed of steatite rocks; 2-way charge/discharge strategy, results for day 5. Inlet temperatures 140°C (charge, 0 to 6 hrs) and 20°C (discharge, 6-12 hrs), initial bed temperature 20°C, air mass flow-rate 0.225 kg/(m^2.s), particle diameter 0.020 m, bed height 3.5 m. The air temperature is shown as a function of depth at each hour.