oa Developing Targets for Power Generation from Multiple Hot Streams

Energy efficiency is growing in importance as governments move to decrease their dependence on fossil based fuels. There are many industries with waste heat streams that need to be cooled down to lower temperatures. The excess heat from these hot streams can be used for generating power using a thermodynamic cycle (such as an Organic Rankine Cycle). A natural first step for assessing such power generation options would to determine the thermodynamic limits that exist for maximum power generation from the single or multiple hot streams. Similar to energy targets from the well-established Pinch Analysis techniques, such power generation targets could inform decision making on pursuing power generation options for the how streams. To date, little work has been done on determining a target efficiency for power generation from multiple waste heat streams. This work is the first to propose two rigorous targeting approaches.

The first approach is based on Carnot efficiencies. As the Carnot cycle is the most efficient power generation cycle, the approach will yield a benchmark in terms of the maximum power that can be generated from the waste heat streams under ideal conditions. The targeting approach will be derived and explained for the general case of multiple hot streams. It will be presented for two common cases of information on multiple hot streams, i.e. the availability of individual hot stream data and the availability of aggregate hot stream data in the form of a composite curve. The Carnot power generation targets cannot be exceeded by real power generation cycles and present the thermodynamic limits for the system. The Carnot targets can be developed very quickly using the proposed approach to inform decisions. The Carnot targets inform the design engineer of power generation potential so that further study can be decided upon (or not) swiftly.

The second targeting approach takes into account the characteristics of real power generation cycles in the form of steam or organic Rankine cycles. The Rankine cycle is chosen in this work as it is the most common thermodynamic cycle employed in for power generation from low grade heat. The proposed approach takes into account the properties of working fluids and a minimum temperature difference between power cycle and the multiple available hot streams. Because of considering the real fluids and driving forces in heat transfer, the targets from the second approach are lower than the Carnot targets from the first approach, but are approachable with real power generation cycles. The real system targets inform the design engineer of power generation potential with Rankine cycles so that further study can be decided upon.

We will focus our presentation on communicating each targeting approach as an easy to use, step by step procedure. Both targeting approaches presented in this work will be illustrated using examples involving power generation from multiple hot streams. We will specifically emphasize the importance of taking a systems view across multiple hot streams when developing power generation systems against multiple hot streams. We will conclude with an outlook on Rankine systems design against multiple hot streams to develop concrete cycle configurations with increased complexity towards the real systems targets.