Note: This was one of the earlier concept we tried for PCM thermal storage. Even though the concept is innovative and can improve the energy density, we decided to abandon it because it was not practical. The benefits of lower use of salt due to higher energy density will be far lower than the cost of freeze protection and safety systems required, as you will see in the write up below.
However, we learned some very important lessons while pumping hot molten salts near their freezing temperature. These lessons are useful for designing systems that use molten salts or molten metals as heat transfer fluid.
Motivation for Research and Conclusions Drawn from Experiments
A major issue preventing the commercial use of Phase Change Material (PCM)-based Thermal Energy Storage (TES) is the difficulty of discharging the latent heat stored in the PCM melt. This is because when heat is extracted, the melt solidifies onto heat exchanger surface decreasing the heat transfer. Thus, to obtain consistently high heat rates, which is required heat rates to generate steam to power a turbine, salt must be removed continuously from heat exchanger surface to improve the heat transfer coefficient and reduce heat exchanger size.
In the past researchers have tried methods to physically remove the salt by scraping it using mechanical scrapers driven using motor and pulleys, impinging a jet of freezing salt on tubes and letting the salt fall-off due to force from the jet, using a ball mill to break the salt from heat transfer surface and other mechanical means. All these methods are commonly used in the industry to make products and perhaps are viable. However, these require supervision, are cumbersome, and high maintenance. For the solar power generation these methods will not work as it requires high maintenance.
Terrafore’s approach described here was to use an ‘anti-stick’ or ‘salt-phobic’ coating on heat exchanger, and alter the morphology to make the freezing salt ‘slushy’ . This will allow use of shell and tube heat exchangers and hopefully low maintenance. This report describes those efforts.
These efforts were successful and Terrafore was able to freeze salt on the tubes and as expected the salt removed by use of hydrodynamic forces and the slurry was pumped back to the tank.
Even though the concept worked, Terrafore determined that there are many engineering risks to scale this system. The benefit of 30% improvement in energy density gained by using phase change does not pay for the additional expense to protect the system using trace heaters for the entire system. Any blockage of salt in the tubes will require expensive repair. Therefore, Terrafore recommended that the development be stopped during Phase 2 of the project.
However, many lessons were learned while experimentally testing this concept by pumping a hot molten salt mixture at near freezing temperatures. These lessons can and were applied to design molten salt systems using conventional 2-tank system (Terrafore supported AEC Engineering with the design of molten salt TES for the 100 MW onopah plant)).
Our innovative approach to solve this problem is to use inorganic salt mixtures that have a simple phase diagram and a mixture with a dilute eutectic composition. A property of dilute eutectic is that when it freezes, it has an equilibrium liquid associated with it. This solid with the associated liquid has the consistency of a slurry, which can be pumped to an external heat exchanger, thus forcing the molten salt over the heat exchanger tubes and causing the freezing solid particles to remain in the molten slurry. The forced convection heat transfer due to flow velocity is significantly larger than conduction through solid. In addition to this, a ‘salt-phobic’ coating on the heat exchanger surface can further prevent the salt from sticking to the tubes and a nucleating agent and chemical added to the salt mixture that can help with the flow and heat transfer properties.
- Dilute eutectic (hyper-eutectic) mixtures of inorganic salts that form a simple eutectic phase diagram exhibit slurry like properties. This property can be taken advantage of to pump salt mixtures over coated heat exchanger tubes and improve heat transfer when PCM is used for storing thermal energy
- Successfully pumped a dilute eutectic molten salt mixture near its freezing point over the specially coated heat exchanger tubes and achieved partial solidification demonstrating use of latent heat of fusion while increasing the heat transfer coefficient. The heat transfer coefficient increased by ten-fold over a passive PCM-TES. However, the solidification achieved was only 15% which is lower than the project milestone of 40%.
- Simulations show that the PCM-TES system can improve the overall system efficiency of the CSP plant by 0.8%, thus requiring about 8% less storage for same capacity factor. This is because for a short period of time, the receiver can be operated to collect energy at lower temperature to melt the salt, which reduces radiation losses from the solar receiver and increases the collection efficiency. This in turn also allows useful energy to be collected during periods of low insolation by running the receiver at lower temperatures.
- With the use of only 15% latent heat in PCM salt, the net specific cost of PCM-TES using the active heat exchanger described in this document is projected to be only 9% when compared to a two-tank conventional sensible storage system. If credit is taken for the improvement in the overall system efficiency, then these savings increase to 17%. The project goal for cost reduction was 30%.
- Even though, these savings are after paying for the added cost of coated tube heat exchanger, there is uncertainty in our estimate of the cost of the parasitic energy for pumping. In addition, since we are pumping molten salt at freezing point, we expect significant issues with designing and operating with heat tracing system to prevent freezing inside piping. Any incident with freeze-up during operation will reduce the savings.
- Significant lessons were learnt when pumping salt mixtures near freezing temperatures through pipe fittings, valves, bends, and pumps. Heat tracing, location and prevention of solidification and high pressures in unwanted areas require special engineering design and care. The failure mode effect analysis, the experienced gained and the designs developed can be used for any sensible or latent heat system where high temperature molten salt is pumped.
Please contact Terrafore for more details.