Efficiencies

Electrochemical (EC) compression is thermodynamically, and inherently, highly efficient. To determine overall system efficiency, one calculates the equilibrium pressure ratio for a gaseous hydrogen concentration cell. This is done with knowledge of the kinetics of both hydrogen reduction and evolution on platinum catalysts, using the Nernst equation. This provides a valid estimate of the equilibrium potential of a hydrogen concentration cell.

Using compressed hydrogen used as a working fluid (ie. the propellant) to compress a second working fluid (ie. the refrigerant). A wide range of refrigerants can be employed under different design/engineering arrangements.

Engaging Hydrogen as the working fluid in a refrigeration cycle is beneficial. Hydrogen is an excellent thermal working fluid with higher thermal conductivity than other gases. In addition, hydrogen has a root-mean-square-velocity 4 to 6 times that of typical refrigerants at room temperature. Because of these two features, hydrogen can readily transfer energy to other working fluids.

In addition, hydrogen gas has much lower viscosity than typical refrigerants. It is useful as an additive to refrigerant gas flowing in a system with lower transport resistance. Hydrogen has excellent EC properties and readily dissociates into protons, which provides for highly efficient gas compression.

Hydrogen gas behaves like a propellant that mixes with the other working fluids and/or refrigerants. In an EC compressor, as hydrogen and some of these fluids enter the MEA, it makes the remaining liquid a little colder. Then on the other side of the membrane, hydrogen gas emerges as a (relatively) warmer gas at higher pressure typically 2 to 8 times higher than the pressure on the cold side. The combined gas mixture is then expanded through an orifice into a larger volume space where it expands and cools down.

It may be desirable to add a complementary process of absorbing the gas stream within the interstices of an activated-carbon bed. This may provide for a single-pressure absorption refrigerator using three substances. This will act as a secondary "pump" for the system augmenting the EC compressor. The activated-carbon can adsorb a large amount of vapors in ambient temperature and desorbs it at a higher temperature.

The system is controlled by moderating the voltage and current being applied to the MEA. In effect the flow of gases is metered to match the required flow of the refrigerant to the load on the evaporator. This will determine the overall capacity of the system, and thus perform the most important function in the overall system.Electrochemical hydrogen compressors have been compared with other compression technologies under practical conditions. Figure 1 provides a comparison of the compression efficiency of hydrogen compressors providing exergetic efficiency greater than 80% and capability to easily attain pressures up to 10,000 psi or 700 bars. This very high exergetic efficiency is the key to the enhanced performance of this device. From cell polarization plots for a low pressure unit (100 psi output), hydrogen pumping rates and actual compression efficiency can be determined at different operating conditions
 
Figure 1 - Efficiency of Hydrogen Compressor
 
It is assumed that all current provides a stoichiometric transfer of hydrogen across the MEA. Transport of water or any other polar-liquid is ignored. Unlike the thermodynamic voltage calculation, this rolls up all losses due to cell resistance and kinetics. Note that, while the actual flow rate of hydrogen cannot be measured accurately by a direct means, it can be measured indirectly from the level of current.
 
An initial, short session was conducted with Professors Stanley Sandler and David Short at the University of Delaware. Using their Aspen Technology Thermodynamic Process Modeling Software (ASPEN Plus V7.1), an analysis was produced. An example of key Input and Process diagrams taken from this initial Aspen Model are shown in Figures 2.
 
 
Figure 2 - ASPEN Thermodynamic Modeling
 
Since EC compression is much more efficient than mechanical compression, the COP is largely a derivative of the fluid mixtures employed and the operating points selected. There is wide latitude in operating conditions, and working fluid mixtures, which have not thus far been fully explored. There is therefore considerable opportunity to improve system operations and efficiency.

It is encouraging that even with early products and modeling efforts for different applications, we have been able to provide COP values roughly 30 to 50% better than mechanical compressors used in those same applications.