375
Characteristics of Small ORC System for Low Temperature Waste Heat Recovery
Musthafah BIN MOHD TAHIR and Noboru YAMADA
Graduate school of Energy and Environment Science, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka-shi, Niigata 940-2188, Japan
E-mail: noborunagaokaut.ac.jp
Abstract
This paper describes fundamental characteristics of a small organic Rankine cycle ORC system to be used for power generation from low temperature heat sources
such as waste heat and solar energy. The aim of the study was to develop an ORC system with a small power output of less than 1 kW with a hot source with
temperature ranging from 60 to 100°C and a cold source with temperature ranging from 10 to 30°C. An ORC system with a potential to produce a turbineexpander
power of 250 W was built and its fundamental characteristics were elucidated. A turbineexpander was not actually installed but was simulated by controlling two
expansion valves. First, steady-state energy balance of the system was examined and the required turbineexpander efficiency was estimated in consideration of
pump power of the working fluid. Then, the relationship between the expansion ratio and thermal efficiency was elucidated. The most important result of the study
was that for maintaining high thermal efficiency in the case that the temperature difference between hot and cold sources varies during operation, it is indispensable
to employ a variable expansion mechanism by which the expansion ratio of the turbineexpander can be adjusted to fit the optimal ratio at the operating
temperature level.
Key words: Organic Rankine Cycle, Thermodynamic Cycle, Thermal Efficiency,
Turbine Efficiency, Expander Efficiency, Pump Efficiency, Heat Exchanger Efficiency, Waste Heat Recovery, Power Generation
1. Introduction
To mitigate the world’s energy problems and global warming, we must use renewable energies. Waste heat is one such renewable energy. In industries around the world, a large
amount of low temperature heat is wasted. According to a report by the Energy Conservation Center of Japan
1
, industrial waste heat in Japan amounts to 2.7 × 10
5
Tcalyear. This amount is equivalent to approximately 70 of the yearly commercial and residential energy consumption in Japan. The report also mentions that the temperature
level of 45 of the total waste heat is 100°C and below. Therefore, it is important to develop an efficient waste heat recovery system to generate power andor electricity from
low temperature heat sources with temperatures of less than 100°C. Further, the size of the recovery system must be fairly small because waste heat is a highly distributed energy
source.
Thus far, various waste heat recovery systems have been proposed and developed. The most feasible and common technique is an organic Rankine cycle ORC, in which a low
boiling point organic fluid is used as a working fluid of the Rankine cycle. For example, Yamamoto et al.
2
described the effect of thermal properties of an organic working fluid on
Received 19 Mar., 2009 No. 09-0115 [DOI: 10.1299jee.4.375]
and Engineering
376
the turbine power output of an ORC system. Freepower Co., Ltd., introduced a commercial ORC system which converts waste heat into electricity
3
. In addition, Yamaguchi et al.
4
invented a unique Rankine cycle system using supercritical carbon dioxide CO
2
as the working fluid, and they elucidated its potential as a solar thermal energy conversion system.
For power generation from a small temperature difference between hot and cold sources e.g. 15 K to 25 K in ocean thermal energy conversion, or OTEC, advanced cycles such as
the Kalina cycle and the Uehara cycle have been developed. Almost all past researches and developments have been carried out for power outputs over 10 kW. For example, turbine
powers of the ORC developed by Ebara Co., Ltd., and Freepower Co., Ltd., are up to 50 kW and 120 kW, respectively; those of OTEC systems and geothermal plants are usually over
30 kW
5, 6
. Accordingly, an ORC with an output of less than 1 kW has not yet been extensively studied and developed. However, the current energy and environmental
conditions worldwide are such that there will soon be a requirement for a small ORC system which can be easily installed close to the location where waste heat is generated.
With the above-mentioned background, the aim of this study was to develop an ORC system with a small turbine power of less than 1 kW, which was equipped with a hot source
with temperature ranging from 60 to 100°C and a cold source with temperature ranging from 10 to 30°C. In this study, an experimental ORC having a potential to produce a
turbineexpander power of approximately 250 W was built and its fundamental characteristics were elucidated. First, the steady-state energy balance of the system was
examined and the required turbineexpander efficiency was estimated in consideration of the pump power of the working fluid. The effect of heat exchange efficiency on the heat
loss of the system was also evaluated. Then, the relationship between the expansion ratio and thermal efficiency was examined and discussed. A theoretical model of the ORC was
employed to calculate the energy balance of the system.
Nomenclature
m : Mass flow rate, kgs
W : Work,
kJkg h :
Enthalpy, kJkg
Q : Heat,
kJkg
Ρ
: Pressure, MPa T
: Temperature, °C or K
P
W
: Pump power, W
T
W
: Turbine power, W
Subscripts C
: Condenser
E
: Evaporator
P
: Pump
T
: TurbineExpander
WF
: Working
fluid
cs
: Condenser inlet
co
: Condenser outlet
cw
: Cold water
hs
: Evaporator inlet
ho
: Evaporator outlet
hw
: Hot
water
pi
: Pump
inlet
po
: Pump outlet
ti
: Turbine
inlet
and Engineering
377
to
: Turbine
outlet
Greek symbols η
R
: Thermal efficiency
η
: Efficiency
γ
int
: Internal heat loss
γ
ext
: External heat loss
2. Theoretical ORC model
