A Knowledge-Based System for the Selection/Design of Low-Grade Waste-Heat Recovery Technology

1. Background

Reducing industrial energy consumption is becoming an increasingly important issue in the UK due to rising energy prices and government legislation which provides a legal obligation to significantly reduce greenhouse gas emissions.

The demand for industrial produce is unlikely to decrease, therefore the emphasis must be on reducing industrial energy consumption by increasing energy efficiency. A key way of doing so is by recovering energy from ‘waste' streams. Unfortunately, many industrial engineers lack the time and/or necessary knowledge to implement waste-heat recovery, particularly when novel technology is required. This often leads to the need for specialist outside consultancy from the start of proposed waste heat recovery projects which may be detrimental to the feasibility due to the high costs involved.

Recent estimates predict that 11.4TWh (Reay and Morrell, 2006) of recoverable low-grade waste-heat (<250^oC) is emitted to the environment each year in the UK process industries. Recovery of this waste-heat could potentially reduce utility bills by up to GBP300M/year and greenhouse gas emissions by 2.1 MtCO₂eq/year (Law et al, 2013)

A number of methods are available to highlight opportunities for waste heat recovery such as pinch technology (Linnhoff, 1983) which has more recently been modified to include more novel methods such as heat pumps and CHP (Klemes and Verbanov, 2012). However, whilst such tools have obvious benefits in highlighting waste heat recovery opportunities, limited methods and tools exist to aid the complex design of the physical heat recovery equipment. This is particularly evident when novel heat recovery technologies such as organic Rankine cycles and heat pumps are required.

2. Aims

This paper presents the development of a knowledge-based system for the selection of low-grade waste heat recovery technology. The system database contains most modern waste-heat recovery technologies, such as heat exchangers (ranging from the common shell-and-tube to compact heat exchangers), heat pumps and organic Rankine cycles.

The system is anticipated to be greatly beneficial to the process industries by addressing the two key barriers to low-grade waste-heat recovery:

i. Awareness of best-available/novel technologies: they are highlighted when suitable.

ii. Cost of consultancy: cost-effective alternative to outside consultancy during the initial stages of waste-heat recovery projects.

3. Methods

Java is used to write the system knowledge-base and GUI, allowing multi-platform utilization of the software and ease of dissemination into the industrial domain.

The software requires a data input from the user consisting of basic, easy to access, properties of the heat source (and any available heat sink) and general plant data. The system then uses the knowledge-base to look up any available options for waste-heat recovery, and standard thermodynamic equations to provide a first design of the technology. The software output consists of an outline equipment design, and the potential economic and environmental benefits of each possible technology. A final choice may be made according to a user-defined criteria (payback time, environmental benefits, capital expenditure etc). A brief overview of the software operation is shown below in Figure 1.

Figure 1. Flowchart depicting overview of software operation

4. Results

The system is tested using four case studies: three UK process industry case studies and one Chinese paper-pulp industry case study (highlighting the opportunity for worldwide utilization of this software). Key results are shown for each possible technology in each case, including a first design, economic assessment and environmental advantages. For example, the key results for a UK food processing case study are shown in Tables 1-3 below.

Table 1. Summary of selection results for UK food processing case study

Heat Source Temperature (o C)	164
Heat Source Nature	Flue gas from cooking oil heaters
Available Heat Sink?	No
Available Heat Sink Within 40oC Temperature Lift?	No
Heat Exchanger Selected?	No
Heat Pump Selected	No
ORC Selected	Yes

Table 2. Summary of ORC design results for UK food processing case study

Working Fluid	R-245fa
Heat Source Inlet Temperature (o C)	164
Heat Source Outlet Temperature (o C)	65.0
Duty of Heat Recovered (kW)	1350
Turbine Inlet Temperature (o C)	110
Turbine Inlet Pressure (bar)	15.4
Turbine Outlet Temperature (o C)	48.0
Turbine Outlet Pressure (bar)	1.48
Working Fluid Mass Flow Rate (kg/s)	5.46
Cooling Water Inlet Temperature (o C)	5.00
Cooling Water Outlet Temperature (o C)	18.6
Cooling Water Mass Flow Rate (kg/s)	20.5
Gross Power Output (kW)	178
Working Fluid Pump Power Required (kW)	8.00
Net Power Output (kW)	170
ORC Thermal Efficiency (%)	12.6

Table 3. Summary of ORC economic and environmental results for UK food processing case study

Units Electricity Generated (MWh/year)	1,430
Equivalent Cost Saving (GBP/year)	149,900
Equivalent Greenhouse Gas Reductions (tCO2eq/year)	748
Estimate Capital Cost (GBP)	455,000
Simple Payback Time (years)	3.04

In this case study, a full energy audit had been completed by the host company and no matching heat sinks were found to be available for transfer of the waste heat via a heat exchanger or heat pump. Therefore, the knowledge-based system suggests that an organic Rankine cycle should be used for waste heat recovery. The current plant engineer had no previous knowledge of organic Rankine cycles for waste heat recovery and therefore would not have reached this conclusion without the aid of the software - this highlights the educational benefits of the program.

Tables 1-3 show that the program has offered a solution which offers great economical and environmental advantages. Firstly, the solution has the potential to reduce utility bills by almost GBP150,000/year, achieving a low project payback time in the region of 3 years which is crucial in the current economical climate. Secondly, the proposed ORC could potentially reduce associated greenhouse gas emissions by up to 748 tCO2eq/year which is highly favorable in the drive towards the tough emissions targets set out by the UK Climate Change Act (2008).

5. Conclusions

A knowledge-based system has been developed to encourage the uptake of low-grade waste-heat recovery projects in the UK processing industries. The system helps to overcome some key industrial barriers to waste-heat recovery.

The system output includes key results for individual case studies including which technologies are suitable for use, an outline design and economic/environmental benefits.

The program has been validated by case-study testing to prove that economically viable and environmentally favorable solutions to waste heat recovery are proposed.

References

HM Government (2008) Climate Change Act. Chapter 27. UK

Klemes, J. J. and Varbanov, P. S (2012) Heat Integration Including Heat Exchangers, Combined Heat and Power, Heat Pumps, Separation Processes and Process Control. Applied Thermal Engineering 43, pp1-6

Law, R; Harvey, A. P; Reay, D. A. (2013) Opportunities for Low-Grade Heat Recovery in the UK Food Processing Industry. Applied Thermal Engineering 53, Issue 2, pp188-196

Linnhoff, B (1983) The Pinch Design Method for Heat Exchanger Networks. Chemical Engineering Science 38, Issue 5, pp745-763

Reay, D. A. and Morrell, M (2007) Overview of Process Heat Recovery, Carbon Trust. Presentation to the Heat Exchanger Action Group (HEXAG). UK