Introduction:

CSPonD is a multidisciplinary project that is looking at the entire system operations of harvesting solar energy, storing it in a molten salt bath, and generating electricity at a later time. During the day the tank would function as a thermal capacitor (storing solar energy). In the evening, the thermal capacitor would be discharged to generate electric power. The molten salt bath is analogous to an electric capacitor. The group is looking at developing tools and bench level prototypes for proving the driving concepts and parameters to successfully design a megawatt size unit. The big picture consists of reflecting solar energy to a reservoir of molten salt, as shown in Figure 1. Individual projects are exploring each phase of system, just to name a few projects: 1) heliostat placement [Corey, et al], 2) heliostat cleaning [Figueredo], 3) volumetric salt receiver [Slocum et al], 4) power cycle, 5) optical attenuation characteristics [Passerini], 6) optical heating [Codd, et al], and 7) energy extraction.

Figure 1 CSPonD basic concept showing a heliostat field reflecting solar energy into a reservoir. The reservoir (shown on the right) is a prototype design for a 4 MWe CSPonD system using 2500 m3 of salt for a period of 40 hour storage (figures courtesy Slocum, et al.).

Motivation:

As the world energy consumption increases, companies and governments alike are looking for renewable forms of energy. However, the problem with most green energy sources, like solar and wind, is that they are cyclical or sporadic. While predictions of energy availability are accurate, currently there is no widely accepted means by which to store large amounts of solar energy over a twenty-four hour period. Concentrated Solar Power on Demand (CSPonD) is a project at MIT, led by Prof. Alex Slocum, whose aim is to receive and store solar energy in the form of a molten salt bath. The molten salt bath can be heated by concentrated solar energy during the day using a heliostat array on the side of a mountain or cliff. Part of the goal is to provide continuous 24/7 power output from the high temperature salt bath. CSPonD can serve as a means to address the growing demand for greener energy sources.

The objective is to develop an accurate thermodynamic model for removing heat from the CSPonD bath to produce electric power. The thermodynamic model created will require validation with experimental data. Once validated, the model will serve as a tool to calculate the effect of changing materials, geometry, fluid properties, etc. on alternative heat exchanger designs and heat transfer for large scale energy storage device.

Abstract:

Three coiled heat exchanger prototypes were designed with containers holding 0.5 kg, 2.3 kg, and 9.2 kg of Sodium Nitrate-Potassium Nitrate salt. The objective was to measure the duration of heat extraction over time for each prototype. The coiled heat exchangers model has been simplified to a flow in a constant wall temperature. The heat extraction model is accurate within an order of magnitude of the measured data. Air is used as the working fluid, with a maximum Reynolds number of 3500 at a maximum flow rate of 10 SLPM. The duration of energy extraction for the first, second, and third prototypes respectively are: 14 min, 29 min, 36 min producing an average energy of 23 W, 23 W, and 21 watts respectively. To compare across the prototypes, the data provided is for bath bulk temperatures starting at 330° C and ending at 275° C. All of the prototypes were left with an open surface free to undergo radiation losses. There is evidence of convection cells in the prototype. A high temperature Particle Image Velocimetry (PIV) system has been proposed to measure the magnitude of the convection cells. Particles in the convection cells are estimated to move between 1 to 3 mm/sec. A proof of concept setup was tested.