(483b) 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

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

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

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