Design considerations for dynamically operated large-scale ammonia heat pumps.

Optimizing control of two-stage ammonia heat pump for fast regulation of power uptake

“…A modulating one-stage R410a heat pump has been shown to be able to react within seconds to a load changing signal, when direct load control is applied and in less than a minute when the load is controlled via the temperature set point [20]. Regulation times for two-stage ammonia heat pumps are expected to be higher, due to the more complex cycle design, which poses challenges with regard to the risk of sudden condensation in the suction line during fast rampdown [21], a large liquid hold-up in the two vessels (intercooler and separator) that results in longer regulation times [22] and time delay of the high-stage compressor, compared to the lowstage compressor according to the control structure (see section 2.3.4).…”

“…The analysed control structures were selected from previously conducted relative gain analysis, where unfeasible structures were excluded. Further, the option of preheating the low-stage suction line was included, as it was shown that the risk of condensation in the suction line during fast ramp down is a problem that is difficult to solve solely based on controller design [21].…”

“…The sum of the actual power uptake of both compressor models at the respective time was the measured input, the set value was the desired power uptake and as output, the required rotational speed of the lowstage compressor was calculated. The control of the evaporation pressure and the source outlet temperature aim at avoiding condensation in the suction line during fast load changes [21] or changes of the thermal boundary conditions. Other possible control structures were disregarded based on the results of a previously conducted relative gain analysis and physical constraints, like e.g.…”

“…Alternatively, the fluid may be superheated before entering the compressor. Different constructive measures to do so were analysed in [21]. These are based on increasing the superheating before the compressor by heating the gas in the suction line or possibly by reducing the pressure.…”