(721d) On the Economics of a Thermal Management System for Industrial-Scale Natural Gas Adsorption Storage

One of the major strategies to lower CO₂ emissions in power generation is to use renewables such as wind and solar photovoltaics. A major challenge to achieving reliable and sustainable supply of power is that renewables are intermittent and require the use of energy storage technologies. Even with modest improvements to current battery technology, electrical energy storage is too expensive. An alternative solution to ensure sustainability of power supply with relatively low CO₂ emissions is the use of stored natural gas. Natural gas storage can ensure a continuous power supply when renewable energy production is insufficient to meet demand. Natural gas can be stored as compressed gas (CNG) at 3000 psig, in liquid phase (LNG) at -162 ^oC, or in adsorbed phase (ANG) at moderate pressure (60 bar) using commercially-available activated carbon adsorbent. CNG and LNG storage technologies are energy intensive processes due to the extreme storage conditions (high pressure and cryogenic temperatures, respectively). ANG storage, however, has the potential to store gas at moderate conditions with a relatively high density. The amount of gas stored using adsorbents is greater than twice that of compressed gas phase at the same operating conditions. The feasibility of ANG storage at an industrial scale critically depends on the cost of the adsorbent material and involved equipment. The adsorption process is exothermic - heat is released when gas is stored in a porous medium, and heat needs to be supplied during the release of the adsorbed gas. Due to temperature changes in the adsorption bed during the adsorption and desorption stages, the usable adsorption capacity is reduced by more than 20%. The adsorption capacity can be maximized by operating under isothermal conditions. Controlling the bed temperature requires the thermal management of the adsorption bed with a heating/cooling system in a shell and tube configuration heat exchanger vessel. As a result, the capital expenditure of the technology is significantly increased compared to running the adsorption/desorption process at non-isothermal conditions and thus a compromise in cost/storage capacity is required. In this work, the economics of an industrial-scale adsorbed natural gas storage (~140 MSCF) for different vessel configurations is explored. The benefits of increasing the usable adsorption capacity via thermal management of the adsorption bed are also addressed. The usable methane storage capacity for the different systems is calculated using dynamic theoretical modeling of the adsorption/desorption cycle. Transient, three-dimensional CFD modeling is used to verify the adsorption bed configuration. The risk adjusted net present value of these systems is estimated to economically compare the gas storage system using a thermal management system, operating at non-isothermal conditions, and storage via compression at the same operating conditions are presented.