More power with Organic Rankine Cycle

The industrial sector in Singapore is the largest energy consumer, accounting for nearly half of the nation’s energy consumption. Over the years, the amount of energy consumed by this sector has grown, and growth is expected to continue as the world recovers from its economic slump[1].

The high energy consumption by the industrial sector presents a considerable potential for energy saving initiatives. In industrial processes with extensive use of heating, drying, evaporating, or others, the application of heat recovery techniques can offer opportunity to reduce energy consumption, running costs, and greenhouse gases emissions.

One of the most useful technologies in recovering waste heat is the Organic Rankine Cycle (ORC), which has found widespread adoption in Europe due to the high energy tariff there. Application of ORC could be a source of opportunity for industries in Singapore to save energy cost and increase the energy efficiency of their operation.

What is the Organic Rankine Cycle?

ORC is a modification of the traditional steam Rankine cycle. The workings of ORC and steam Rankine cycle is indistinguishable, comprising of a four-stage heat engine thermodynamic cycle; the difference being the replacement of water as the working fluid in the steam Rankine cycle with an organic refrigerant in ORC. Here, organic means compounds made up of carbon, hydrogen, and oxygen.

Figure 1: Schematic of the Organic Rankine Cycle

In ORC, an organic working fluid is first pumped from a lower pressure to a higher pressure. Then the high pressure fluid enters a heat source where it is heated at a constant pressure until it becomes a dry saturated vapour. Next, this vapour expands through a turbine where mechanical work is produced and converted to electrical energy. The wet vapour enters a condenser where it is condensed back into a saturated liquid and the cycle starts again.

Application of ORC in industries

As ORC operate under a lower operating temperature and pressure than in traditional steam Rankine cycle, this allows a lower grade heat to act as fuel for operations. Low grade heat refers to low-and mid-temperature heat with low energy density that cannot be converted to electrical energy efficiently by traditional steam Rankine cycle due to the higher operating temperature and pressure requirements. This creates a wider range of application for ORC which would be unfeasible with steam Rankine cycle.

Here, we will look at two common applications of ORC for waste heat recovery in the industrial sector.

As a bottoming cycle in cogeneration:

ORC systems can aid in recovering heat for gas turbines in cogeneration systems, offering advantages over traditional steam Rankine bottoming cycles. A relatively new application being explored as an appropriate implementation of ORC is using it as a bottoming cycle at cogeneration plants. This can lead to an increase in system efficiency by as much as 15%.

For recovery of low-grade waste heat:

This area of application is probably where ORC is most useful. One major application is in process heat applications, such as kilns and dryers. ORC system sizing varies greatly, ranging from tens of kilowatts to as large as several megawatts. Regardless of size, the real importance of ORC is ability of the system to convert low- grade waste heat to power. While using steam Rankine cycle to recover energy off low-grade waste heat source would be very inefficient and expensive, using ORC would make the process economically feasible and worthwhile. ORC can even be applied for waste heat at temperatures Figure 2: An ORC system for waste heat recovery as low as 70oC.

Other advantages of ORC

Besides the ability to apply ORC to recover waste heat normally inaccessible to traditional steam Rankine cycle, there are other advantages accorded by ORC.

Lower maintenance cost: As ORC systems operate at lower temperature and pressure and have limited moving components; the cost of maintenance is reduced – to only a fraction of comparable fossil-fuel generators.

Minimal supervision required: Due to the lower operating pressure, this generally eliminates the need for an operator to monitor ORC systems. Most systems come with computerised remotely monitored control units. Greater equipment longevity: As ORC systems operate at lower pressure and turbine speed, the mechanical stresses on the equipment are lower. Also, by replacing water with organic fluid as the working fluid, moisture responsible for turbine blades erosion during vapour expansion is eliminated.

ORC has proven to be a useful and reliable technology in the recovery of waste heat in industrial applications, with distinct advantages over traditional steam Rankine cycle. The potential for ramped-up application of ORC is great, as it is likely that companies with expertise in chillers and refrigeration systems will have the skill set to build high quality, high performance ORC systems due to the common refrigerant used. A wider application of this technology will lead to energy and other cost benefits to industrial companies, as well as greater energy efficiency in the industrial sector.

To learn more about Organic Rankine Cycle in the industrial sector, please visit: http://aceee.org/files/proceedings/2011/data/papers/0085-000077.pdf

Contributed by H2PC Asia resource team of E2 writers. Please contact [email protected] ______

[1]. ‘Energy Efficiency in Singapore’s Industrial Sector’ http://esi.nus.edu.sg/publications/2012/02/02/energy-efficiency-in-singapore-s-industrial-sector