Supercritical CO2 Power Cycle Based on Solar Thermal Energy and Ammonia Absorption Chiller

by Jitender Kumar
Freelance Consultant

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This paper is about a theoretical analysis for an SCO2 cycle based on availability of a heat source (T > 500oC), which in the current analysis is assumed as concentrated solar thermal energy, with steam as an intermediate fluid that heats the CO2. The cycle analyzed is based on use of steam based ammonia absorption chilling system for condensing the low pressure CO2. It assumes availability of the refrigerant at <= -40oC, since the CO2 sub-cooling temperature has been taken as -33.8oC, condensing at about 291 psia. So, there are two components of the heating duty for this cycle –
  1. high grade steam (say, 550oC) for heating the power cycle fluid, CO2, and
  2. relatively lower grade steam (about 175oC) for the chiller.
Since the chilling load in the basic cycle, with the conditions considered in this analysis, is a significant component, about 58.4% (for coefficient of performance, COP = 0.4) of the overall thermal energy input, and the cycle had a significantly lower overall efficiency, kWe(net)/kWt, compared to a steam based Rankine cycle, and to an SCO2 Brayton cycle, further analysis was done for any possible improvements.

The following options were analyzed for improvements -
  1. An option for heat integration between the chilling cycle[1] and the SCO2 cycle. This was done by splitting the ammonia flow/cooling load between the chilling circuit condenser and the CO2 cycle.
  2. An integration of a supercritical steam based power plant and an SCO2 based power plant, where both are supplied high grade heat from a single source. The integration was done by using lower grade steam extracted from the steam extraction based cycle, operating entirely in extraction mode, for two purposes –
    • operating the absorption chiller, and
    • crediting some low grade heat to the SCO2 cycle, thereby indirectly reducing the latter’s high grade heat input, essentially shifting some of its heating duty to the steam based cycle.
  3. Increasing the highest temperature in the SCO2 cycle to 800oC – this assumes solar heat based input merely for presenting the idea. The heat source may be different, possibly a fossil fuel.
These were compared, for net efficiency (kWe/kWt), with a separate supercritical steam (Rankine) cycle, in each case. The details are in the Summary & Conclusions section, Table 8.

The analysis is based on the following –
  1. Review of existing literature on the SCO2 cycles [2]. Most of the data, and some of the operating conditions were taken from the references, e.g. CO2 condensing conditions [3], absorption cycle ammonia condenser conditions [1]. The analysis can be improved with involvement of system component suppliers.
  2. Simplifications, e.g. pressure changes across power cycle equipment, except across CO2 pump and turbine, have mainly been ignored. Minimum temperature difference in heat exchangers, at any end, has been taken as 10oC, and about 8oC for CO2-steam heat exchanger. Practically, a higher value of the former may be a better choice.