Onboard Refrigeration with Ammonia

The presence of ammonia on board offers the opportunity to use it as a refrigerant for engine cooling. Moreover, ammonia can be used as working agent to recover engine heat and convert it into some additional shaft work. Optionally, the refrigeration effect of ammonia, i.e., its high latent heat of vaporization, can be used to produce some air conditioning on board.

The cooling effect of ammonia is equivalent to the heat needed to raise its temperature plus, if it applies, the heat needed to decompose it (partially or totally) to produce hydrogen. Expressed in terms of enthalpy this heat is

where T represents the temperature at which NH3 is usually decomposed, xd represents the fraction in which the ammonia stream is dissociated into H2 and N2 (if this applies), nd is the efficiency of the decomposition unit, which based on the results by Yin et al. (2004) is assumed here to be 0.9, and Ahd represents the dissociation heat at T. Note that in some applications, for instance, fueling internal combustion engines, there is no need to completely decompose ammonia into hydrogen, because these systems may operate with a mixture of ammonia and hydrogen. The decomposition takes place according to the following reaction:

The heat of this endothermic reaction has been calculated here as a function of the temperature based on Shomate equation as implemented by the NIST (2008). Recent developed technologies operate reaction (5.8) at temperatures in the range of 300-700oC, respectively.

In order to quantify the cooling effect of ammonia in relative terms we introduce here a cooling effectiveness, defined through the heat given by Eq. (5.7) and the lower heating value (LHV) of ammonia:

The results of applying Eq. (5.9) for a range of ammonia reforming temperatures and various decomposition fractions are presented in Fig. 5.6. The thermodynamic data for plotting Fig. 5.6, as well as throughout this chapter, are calculated using the FluidProp software developed by Colonna and Van der Stelt (2004).

The case xd=0% represents the hypothetic situation where ammonia is only preheated prior to oxidation and one assumes that no decomposition occurs. This case is illustrated for reference, because in reality, at temperatures over 300oC some ammonia decomposes spontaneously, even without the presence of catalysts. The case for which xd=5% is applicable to some internal combustion engines, where as discussed above, a small fraction of ammonia is usually decomposed to produce hydrogen that boosts the combustion process. The extreme situation when xd=100% is applicable to some fuel cell systems that are supplied with pure hydrogen produced from ammonia.

Fig. 5.6 Cooling effect of NH3 vs. the decomposition temperature and for several decomposition fractions.

Fig. 5.6 Cooling effect of NH3 vs. the decomposition temperature and for several decomposition fractions.

The results in the above figure show that the maximum achievable engine cooling with ammonia represents slightly over 20% of the LHV. Thus, in power systems using hydrogen than ammonia, the usual water-cooling system may be downsized with up to 20%. Optionally, a part of this cooling may satisfy some air-conditioning needs on the vehicle.

With reference to Fig. 5.7, one assumes that the saturated liquid is extracted from the thermally insulated fuel tank. The liquid stream can be throttled such that the fuel evaporation is conducted at the desired temperature (e.g., 5-10oC will suffice either for engine cooling or for obtaining some air-conditioning). After throttling, the fluid passes through a heat recovery heat exchanger where the engine coolant is cooled with ammonia. If air-conditioning is desired, the heat recovery will take place is two steps: first the air and subsequently the engine's coolant are cooled with ammonia.

In order to give a numerical example, let us assume the temperature in the fuel tank to be 25oC, the evaporation temperature to be 5oC, and the ammonia temperature at the evaporator to be 15oC (superheated vapor). With these figures, the cooling effect is quantified at 6.3% of the LHV of ammonia. This means that for a medium size car equipped with 70 kW engine while the engine runs on H2 than ammonia at full load the obtained refrigeration effect to be used in the form of air-conditioning amounts to ~4.4 kW. In top of this, up to 15% LHV meaning 10.3 kW is available for engine-cooling purpose. Alternatively, ~15 kW can be made available for engine cooling only.

Cooling Liquid Meaning

One important remark is that while leaving the tank the liquid takes-out its flow enthalpy. This enthalpy is replaced by evaporation of the corresponding quantity of liquid in a manner that the temperature and pressure in the tank could be maintained constant. A coil can be immersed into the liquid through which air and coolant circulate. The coolant is subject to cooling while ammonia is consumed from the tank, and this cooling effect can be exploited for any useful need on board. The equation h' (T )mNHs = mair (hin - hout) represents the energy balance for the cooling coil, where the LHS is the enthalpy of the leaving liquid and RHS is the enthalpy change of the coolant (e.g., air).

The improvement of engine efficiency due to the associated cooling effect of ammonia can be quantified based on the typical coefficient of performance (COP) of the vehicular cooling systems. The gain in work at the engine shaft due to the available cooling from ammonia (i.e., the one that comes from fan, pump, and compressor power savings) is

and induces an engine performance improvement that can be quantified by the effectiveness introduced below

For an assumed (typical) COP of 2 (COP of the engine-cooling system and air-conditioning system at an average), the maximum gain in efficiency is about 10%. It is to be remarked that the simplicity of this cooling system (that consists only of one or two heat exchangers and one throttling valve) lowers both the initial, operation, and maintenance costs by eliminating or downsizing the conventional mechanical cooling system (that comprises the compressor, condenser, water pump, fan, and radiator).

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