Large-Scale Pumped Thermal Electricity Storages—Converting Energy Using Shallow Lined Rock Caverns, Carbon Dioxide and Underground Pumped-Hydro

A fast-paced energy transition needs a higher penetration of renewables, of heating and cooling in the worldwide energy mix. With three novelties 1-of using shallow high-pressure LRC (Lined Rock Cavern) excavated close to storage needs, 2-of using a slow-moving CO2 piston applying steady pressure on the hydro part of UPHES (Underground Pumped Hydro Energy Storage) and 3-of relying on inexpensive thermal stores for long-duration storage, CO2 UPHES coupled with PTES (Pumped Thermal Electricity Storage) could become, at expected Capex cost of only 20 USD/kWh electrical, a game-changer by allowing the complete integration of intermittent renewable sources. Moreover, even though this early conceptual work requires validation by simulation and experimentation, CO2 UPHES as well as UPHES-PTES hybrid storage could also allow a low-cost and low-emission integration of intermittent renewables with future district heating and cooling networks.

Numerical Investigation of Entropy Generation in Stratified Thermal Stores

This paper investigates the nature of entropy generation in stratified sensible thermal energy stores (SSTES) during charging using a dimensionless axisymmetric numerical model of an SSTES. Time-varying dimensionless entropy generation rates and the cumulative entropy generation in SSTES were determined from finite volume computations. The aspect ratios (AR), Peclet numbers (PeD), and Richardson numbers (Ri), for the stores considered were within the ranges 1≤AR≤4, 5×103≤PeD≤100×103, and 10≤Ri≤104, respectively. Using the Bejan number (Be), the total entropy generation was shown to be almost entirely due to thermal effects in the SSTES. The Be is practically unity for most of the SSTES' charging duration. The contributions of radial thermal gradients to the thermal entropy generation were further shown to be largely negligible in comparison to the contributions of axial thermal gradients, except at low Ri. Entropy generation numbers, Ns, in the SSTES were also computed and found to increase with decreasing AR and PeD and with increasing Ri. PeD was found to have the most significant influence on Ns. Based on this axisymmetric analyses of time-varying entropy generation in SSTES, estimates have been obtained of (1) the relative significance of radial effects on entropy generation within SSTES and (2) the relative significance of viscous shear entropy generation mechanisms within SSTES.

Use of a Shroud and Baffle to Improve Natural Convection to Immersed Heat Exchangers

Optimizing heat transfer during the charge and discharge of thermal stores is crucial for high performance of solar thermal systems for domestic and commercial applications. This study models a sensible water storage tank for which discharge is accomplished using a heat exchanger immersed in the storage fluid. The heat exchanger is a two-dimensional isothermal cylinder in an adiabatic enclosure with no initial stratification. An adiabatic shroud and baffle whose geometry is parametrically varied is placed around and below the cylinder. Transient numerical simulations of the discharge process are obtained for 105 < RaD < 107, and estimates of the time needed to discharge a given fraction of the initial stored energy are obtained. We find that a short baffle is least effective in increasing heat transfer rates. The performance benefit is greatest early in the transient discharge period when the buoyant flow in the store is strongest. As with all flow control devices, the benefit decreases as energy is extracted from the tank and the temperature difference driving the flow decreases. The use of a shroud increases the transient Nusselt number by as much as twentyfold.

Negatively Buoyant Plume Flow in a Baffled Heat Exchanger

A numerical simulation of transient two-dimensional negatively buoyant flow into a straight baffle situated below an isothermal circular cylinder in an initially isothermal enclosure is presented for both an adiabatic and a highly conducting baffle for Rayleigh numbers from 106 to 107. Results show the effects of baffle offset, width, and length on the point where viscous flow develops and on velocity profiles within the baffle. Results are interpreted to guide the design of straight baffles to reduce destruction of stratification in thermal stores using an immersed heat exchanger. The preferred geometry is a low-conductivity baffle of width equal to the effective width of the heat exchanger and 15 or more cylinder diameters in length to ensure nearly fully developed flow at the baffle outlet.