Making more of waste heat: Introducing ORC

Written by: RWW | Published:

DECC estimates that between 10 and 40 terawatt hours of heat is currently lost and rejected from industrial sources in the UK every year. More of it could be recovered, but where there is no local demand to exploit the heat directly, an alternative is to use it to generate electricity. Trevor Smith reports on a new technology.

Equipment using a relatively unknown technology called the Organic Rankine Cycle (ORC) is now commercially available to recover waste heat. ORCycle is a company that is actively seeking to identify opportunities and promote the use of ORC technology in the UK and is working with Belgian manufacturer, E-Rational, who can supply 500kW, 1MW and 4MW thermal rated generators as single modules or built-up as an array for larger capacity.

Both waste heat and heat from renewable sources can be used as the ‘fuel’ for the ORC generator. As a guide, the temperature needs to be in the range 80 to 150 °C and in the form of steam, hot water or hot thermal oil. Applications are found in fields as diverse as:

Anaerobic digesters producing methane which are used to power internal combustion engines to generate electricity. Heat from the engine exhausts can be used to produce hot water, used for maintaining the digesters at the optimum temperature. Surplus hot water can be used in the ORC generator to produce additional power

Landfill sites generating electricity from landfill gas could similarly increase their electrical output by retrofitting heat exchangers to the exhausts and circulating water, heated by exhaust gases, through the ORC generator

Deep geothermal sources at a few exceptional locations in the UK can provide water hot enough to produce electricity by ORC

Biomass combined heat and power (CHP) plants producing electricity and surplus heat - perhaps because of restrictions or seasonal variations in demand

High temperature incineration poses a number of technical challenges to recovering heat from the flue gases but nevertheless represents an abundant source of good quality heat

Energy from waste (EfW) with access to low pressure steam, an electricity grid connection and a cooling source could achieve an attractive return on investment for the marginal cost of installing an ORC generator.

How it works

Most electricity generation in the world is produced from thermal sources of some type and involves the conversion of heat to work using a working fluid, which cycles through heat adsorption, work production and heat rejection. Thermodynamics provides theoretical rules for maximising the efficiency of the conversion such as ‘the hotter the heat source and the colder the cooling medium, the higher the overall efficiency will be’ and ‘ideally, all heat transfer should occur at constant temperature’.

The Rankine cycle is nearly ideal because much of the heat transfer occurs during boiling or condensing the working fluid - conventionally steam. Power is produced by boiling a pressurised liquid, expanding the high pressure vapour to produce work, and then condensing low pressure vapour and pumping condensed liquid back to the boiler.

As a working fluid, steam has advantages and disadvantages. 

Water is cheap, readily available and non-flammable. But steam plants cannot take full advantage of the low ambient temperature of a climate such as the UK because the very low pressure the steam expands to increases the size of the equipment to an impractical scale. And the relatively high boiling point of water limits the opportunity for recovering heat much below 150° C. The idea of replacing steam with an organic fluid has been around for 100 years, but only now is gaining recognition in the UK as a green technology.

Organic fluids can overcome the disadvantages of steam. E-Rational equipment uses environmentally-friendly refrigerants like R245fa which would boil in the palm of your hand, clearly demonstrating its potential for extracting useful energy from low grade heat sources. Having both higher molecular weight and vapour pressure than water, R245fa is around 100 times as dense as steam, making ORC equipment compact comparable to steam plant.

As high pressure R245fa passes through an expansion device, no condensation takes place - unlike steam which, unless superheated, forms water droplets that erode turbine blades. 

The need for superheating is an additional cost and further limits the usefulness of steam to recover low-grade waste heat. ORC fluids simply do not have this problem.

The ORC advantage in energy from waste

A typical EfW plant has boilers producing high pressure steam, which expands through condensing steam turbines that generate electricity. 

Fully expanded, steam condenses at perhaps 60°C, under vacuum and at very low density, in an air-cooled condenser. Accordingly, the condensers are a monumental size.

Condensing turbines frequently have the facility to remove steam at low pressure from a port in the casing, after it has delivered most of its potential work in the high pressure section of the turbine but before it has fully expanded. This ‘bleed’ steam is used within the facility for boiler feed water preheating and can be used as the heat source for CHP and district heating schemes.

SELCHP (South East London Combined Heat and Power) recently completed £5m worth of distribution pipework for CHP demonstrating how much district heating can cost, even in a densely populated area like South East London. 

ORC generation offers an alternative and complementary use for low pressure steam at EfW plants that are already producing bleed steam or are designed to be ‘CHP ready’ and have the capability to draw off this steam. Consider:

Bleed steam is available, but there is little or no local demand for the heat

There are insufficient capital funds for the district heating scheme

District heating is supplied, but there is a seasonal lack of demand for heat.

In all these cases, ORC can be used to generate additional electricity from bleed steam. A bonus to operating this way is heat loading to the air cooled condenser is reduced by the amount of steam condensed in the ORC - lowering the condenser temperature and increasing overall plant efficiency. 

Great potential for new build

Intriguingly, even greater efficiencies and capital cost savings may potentially be achieved by integrating ORC at the design stage of future EfW plants. If condensing turbines were replaced with back pressure turbines the entire output of low pressure steam could feed into a bank of ORC generators. 

In effect the heat would be used twice, first in the steam Rankine cycle and then in the ORC. This so-called ‘bottoming cycle’ (though yet to be built in the UK) is a promising technology for next generation EfW.

ORCycle believes that substantial savings in energy use and operating costs are achievable by introducing the efficient, cost effective, simple to operate ORC system. In short, we think it’s a waste to waste waste heat. 

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