Performance characteristics of packed bed thermal energy storage for solar thermal power plants

Allen, Kenneth Guy (Stellenbosch : University of Stellenbosch, 2010-03)

Thesis

ENGLISH ABSTRACT: Solar energy is by far the greatest energy resource available to generate power. One of the difficulties of using solar energy is that it is not available 24 hours per day - some form of storage is required if electricity generation at night or during cloudy periods is necessary. If a combined cycle power plant is used to obtain higher efficiencies, and reduce the cost of electricity, storage will allow the secondary cycle to operate independently of the primary cycle. This study focuses on the use of packed beds of rock or slag, with air as a heat transfer medium, to store thermal energy in a solar thermal power plant at temperatures sufficiently high for a Rankine steam cycle. Experimental tests were done in a packed bed test section to determine the validity of existing equations and models for predicting the pressure drop and fluid temperatures during charging and discharging. Three different sets of rocks were tested, and the average size, specific heat capacity and density of each set were measured. Rock and slag samples were also thermally cycled between average temperatures of 30 ºC and 510 ºC in an oven. The classical pressure drop equation significantly under-predicts the pressure drop at particle Reynolds numbers lower than 3500. It appears that the pressure drop through a packed bed is proportional to the 1.8th power of the air flow speed at particle Reynolds numbers above about 500. The Effectiveness-NTU model combined with a variety of heat transfer correlations is able to predict the air temperature trend over the bed within 15 % of the measured temperature drop over the packed bed. Dolerite and granite rocks were also thermally cycled 125 times in an oven without breaking apart, and may be suitable for use as thermal storage media at temperatures of approximately 500 ºC. The required volume of a packed bed of 0.1 m particles to store the thermal energy from the exhaust of a 100 MWe gas turbine operating for 8 hours is predicted to be 24 × 103 m3, which should be sufficient to run a 25-30 MWe steam cycle for over 10 hours. This storage volume is of a similar magnitude to existing molten salt thermal storage.

AFRIKAANSE OPSOMMING: Sonenergie is die grootste energiebron wat gebruik kan word vir krag opwekking. ‘n Probleem met die gebruik van sonenergie is dat die son nie 24 uur per dag skyn nie. Dit is dus nodig om die energie te stoor indien dit nodig sal wees om elektrisiteit te genereer wanneer die son nie skyn nie. ‘n Gekombineerde kringloop kan gebruik word om ‘n hoër benuttingsgraad te bereik en elektrisiteit goedkoper te maak. Dit sal dan moontlik wees om die termiese energie uit die primêre kringloop te stoor, wat die sekondêre kringloop onafhanklik van die primêre kringloop sal maak. Dié gevalle studie ondersoek die gebruik van ‘n slakof- klipbed met lug as hitteoordragmedium, om te bepaal of dit moontlik is om hitte te stoor teen ‘n temperatuur wat hoog genoeg is om ‘n Rankine stoom kringloop te bedryf. Eksperimentele toetse is in ‘n toets-bed gedoen en die drukverandering oor die bed en die lug temperatuur is gemeet en vergelyk met voorspelde waardes van vergelykings en modelle in die literatuur. Drie soorte klippe was getoets. Die gemiddelde grootte, spesifieke hitte-kapasiteit en digtheid van elke soort klip is gemeet. Klip en slak monsters is ook siklies tussen temperature van 30 ºC en 510 ºC verkoel en verhit. Die klassieke drukverlies vergelyking gee laer waardes as wat gemeet is vir Reynolds nommers minder as 3500. Dit blyk dat die drukverlies deur ‘n klipbed afhanklik is van die lug vloeispoed tot die mag 1.8 as die Reynolds nommer groter as omtrent 500 is. Die ‘Effectiveness-NTU’ model gekombineerd met ‘n verskeidenheid van hitteoordragskoeffisiënte voorspel temperature binne 15 % van die gemete temperatuur verskil oor die bed. Doloriet en graniet klippe het 125 sikliese toetse ondergaan sonder om te breek, en is miskien gepas vir gebruik in ‘n klipbed by temperature van sowat 500 ºC Die voorspelde volume van ‘n klipbed wat uit 0.1 m klippe bestaan wat die termiese energie vir 8 ure uit die uitlaat van ‘n 100 MWe gasturbiene kan stoor, is 24 × 103 m3. Dit behoort genoeg te wees om ‘n 25 – 30 MWe stoom kringloop vir ten minste 10 ure te bedryf. Die volume is min of meer gelyk aan dié van gesmelte sout store wat alreeds gebou is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/4329