(460b) Optimization Opportunities for Stand-Alone Liquid Air Energy Storage

Our energy systems are likely to follow two major trends in the coming years. First, there is the obvious and imperative (from an environmental point of view) transition from fossil fuels to various renewable energy forms. Second, there is a trend from very large centralized industrial energy systems to decentralized energy hubs serving both industry, transportation and the domestic sector. With time variations (daily and seasonal) both on the supply side (intermittency of renewables) and the demand side, energy storage technologies will play a very important role. A large variety of such energy storage systems exist, however, these technologies span from quite mature to emerging states. The two main categories are thermal and electrical energy storages, and examples include sensible heat, latent heat, thermo-chemical, pumped hydroelectric, batteries, compressed air and liquid air. The performance of these energy storage technologies is typically evaluated by using a number of parameters and properties, such as round-trip efficiency, energy density, energy capacity, geographical constraints, capital intensity, space or weight intensity, etc.

Liquid Air Energy Storage (LAES) uses surplus of power to run compressors as an important element together with cooling in the liquefaction of air. When power is in demand, liquid air is pumped, evaporated and expanded to produce power. There is a considerable literature on both liquefaction and separation of air, and such technologies have been available for more than 100 years. Examples are the Linde-Hampson (US Patent, 1903), Claude (US Patent, 1913) and Kapitza (US Patent, 1952) processes. When using liquid air as a way to store surplus power, however, the regasification (discharging) part is equally important as the liquefaction (charging) part. Energy recovery (power, heat and cooling) between the charging part and the discharging part is crucial in order to achieve an acceptable round-trip efficiency, i.e. the ratio between power produced in discharging step and power consumed in charging step. A recent LAES concept by Guizzi et al. (2015) introduced three thermal energy storages (one hot and two cold) between the liquefaction and regasification processes to significantly improve the round-trip efficiency to 54.4%. Their system can be referred to as a modified Claude process.

This work studies different ways to improve the basic scheme suggested by Guizzi et al. (2015) by considering structural changes, operating conditions, integration opportunities between the charging and discharging parts, and internal waste heat recovery. A major challenge is the fact that the charging and discharging processes for obvious reasons operate at different times. Air is liquefied when there is surplus of power and then regasified when there is demand for power. From an energy balance point of view, this is taken care of by the three thermal energy storages, but the optimization of the energy storage fluids (flowrates and temperatures) is more complicated than if the two parts of the LAES had been operated simultaneously. There are, however, a number of additional challenges that need to be addressed in order to optimize this energy storage technology. A considerable number of degrees of freedom in the LAES system can be used to improve its energy efficiency, primarily defined by the round-trip efficiency, and these issues will be discussed here:

• Number of compressor stages with interstage cooling
• Number of expander stages with interstage reheating
• Identification of appropriate fluids for the thermal energy storages
• Using Organic Rankine Cycles to produce power from internal waste heat

As an example, consider the operation of the compressors for the incoming air. Isothermal operation (which of course is not practical) minimizes compression work. This ideal operation can be approached by increasing the number of compression stages with cooling of air between the stages. As a result, both the amount and quality (temperature) of the heat that can be delivered to the hot storage and subsequently to the expansion process are reduced. Saving power in the compression part will therefore reduce power production in the expansion part of the system. Adiabatic compression, on the other hand, results in maximum compression work and therefore considerable amounts of heat at high temperature that can be transferred to the expansion section and thereby increase power production. Obviously, the operation of the compression and expansion part (primarily the number of stages) is linked through the fluid of the hot storage. Thus, there is an optimal combination of compressor and expander stages, and the round-trip efficiency will be affected.

Through a combination of topological improvements (e.g. varying the number of compression and expansion stages), improved integration between the charging and discharging parts, optimization of operating conditions (by using a Particle Swarm Optimization algorithm), and using Organic Rankine Cycles for converting internal waste heat to power, the round-trip efficiency of the LAES has been improved from 54.4% to 63.8%, as illustrated in Figure 1. In our work, we have challenged the selection of fluids for thermal energy storage used by Guizzi et al. (i.e. hot oil, methanol and propane). More specifically, for the two cold storages, we have analyzed the use of mixtures to replace the pure component fluids. Case 1 in Figure 1 is the base case for our research as defined by Guizzi et al. (2015).

There is considerable scope for further improvements of the LAES system, in particular the potential integration of external waste heat and external cold energy. Capital cost is also an important topic for future research on the system, and will not be addressed here.

Figure 1. Improvement opportunities in Round-Trip Efficiency for the LAES.

References:

Claude G. Process for separation of the constituents of gases mixtures, US Patent no. 1,068,219, July 22, 1913.

Guizzi G.L., Manno M., Tolomei L.M. and Vitali R.M. Thermodynamic analysis of a liquid air energy storage system, Energy, 93:1639-1647, 2015.

Kapitza P.L. Method and means for distillation of low boiling point liquids, US Patent no. 2,608,070, 26 August, 1952.

Linde C. Process of producing low temperatures, the liquefaction of gases, and the separation of the constituents of gaseous mixtures, US Patent no. 727,650, May 12, 1903.