1. ORC Unit: Organic Rankine Cycle is a power generation Plant; on a mini-scale; typically in the range of 10 to 250 kW. The main differences between the large thermal power plants which operate on conventional steam based Rankine cycle and ORC are:
a) Working fluid is an Organic fluid; typically a refrigerant e.g. R245fa..
b) The temperature levels are low; for example hot water @ an inlet temperature of 90 C can be used as the heat source.
c) Instead of a conventional turbine, an Expander is used here. Different designs of Expanders are used; notably Screw type expander or Scroll expander.
d) Since the heat source temp levels are low, the conversion efficiency is low; typically 8 to 10 %.
Effectively Organic Rankine Cycle is the reverse of Vapour Compression Refrigeration cycle.
2. The Idea: An ORC Unit needs a cooling water source for rejecting the heat in the cycle. Now, considering Indian climatic conditions which are representative of tropical conditions, the cooling water from a Cooling Tower would usually be available at @ 30 to 32 C. As per 2nd Law of thermodynamics, the larger the difference between the source ( in this case the hot water ) temperature and the sink i.e. cooling water temperature, the greater will be the efficiency of conversion and thus one can get higher power output. The proposed new idea targets to reduce the cooling water inlet temperature. This can be done as follows :
a) It is assumed here that the power generation is taking place only during daytime i.e. the heat source availability ( for example solar energy) is only during daytime.
b) In many places in India there is a large difference between the daytime high temperature and night time low temperature.This natural phenonmenon can be used to reduce the daytime cooling water temperature by employing the concept of Thermal Energy Storage (TES) using PCM’s (Phase Change Materials). There are some commercially available PCM’s which have a melting ( and freezing ) temperature in the range of 15 to 25 C. So the idea involves storing the night time coolness into these PCM’s and then using that cold energy during the day time to provide lower temperature cooling water for the ORC Unit. It can be estimated that we can get cooling water temperature as low as 20 C at some times of the year. In other seasons it may go up to 25 C; however still it will be advantageous for the ORC Unit which will operate at a higher conversion efficiency.
While it needs more design and engineering calculations to estimate accurately the efficiency increase, as a rough estimate it can range from 2 to 5 % which will be good considering the fact that the basic conversion efficiencies of these Units is itself low.
The system will be an engineered system consisting of TES tanks, encapsulated PCM’s ; the cooling water piping, pumps, valves , instrumentation and a control system.