The organic rankine cycle revolution

A quiet revolution is taking place in converting exhaust heat from power-generating engines into 10% additional power using Organic Rankine Cycle (ORC) technology, writes Jos van Buijtenen

The payback period for an ORC can be as short as three years Credit: Triogen

Turbochargers developed several decades ago from exotic devices on sophisticated ships and high-performance engines in to a standard feature of regular engines today, with their benefits taken for granted. A similar, quiet revolution is now taking place in converting exhaust heat from power-generating engines into 10% additional power; the technology is well proven and payback can be as short as three years.

Recently, new concepts for power generation and efficiency increases are under investigation, with the aim of reducing generation costs and satisfying rising energy demand within a decentralized production approach. Many industrial processes only use 30%-40% of their primary energy input. As a result, more than half of the primary energy used worldwide in the industrial and agricultural sectors is lost as waste heat. The use of this waste heat by means of new technological alternatives for turning it into useful energy is gaining attention as a sustainable source of clean and reliable power. Besides economic drivers, social and environmental benefits are also becoming part of investment decisions.

Corporate interest in reducing energy consumption and carbon footprint levels is increasing. Utilizing heat-to-power technologies allows industry to generate their own emission-free power from a resource they already own, reducing their operating costs and carbon footprint.

By reducing the energy consumed from the grid, power from otherwise wasted heat reduces emissions from fossil fuel generation and avoids losses through transmission and distribution.

Figure 1. Schematic view of a Triogen ORC plant

ORC technology

The Organic Rankine Cycle (ORC) uses heat to produce electricity and is the best solution when the temperature of the heat source is relatively low, or where the size of the application is too small for a steam power plant to be the most efficient solution. Large ORC plants have been operating on geothermal heat sources or multi-MW biomass combustion plants for many years. This technology is supplied by publicly listed companies such as Ormat and Turboden, a subsidiary of Mitsubishi. In addition, the ORC technology is establishing itself as a well-proven application in small, decentralized power plants.

The working principle of an ORC plant is based on the same process as traditional water steam cycles used in conventional power plants for 150 years. Using water as a working fluid is state-of-the-art in nuclear- and coal-fired power plants. However, exchanging the working fluid with an organic medium with a lower evaporation point enables application of the technological concept also in small- to medium-scale applications with relatively low temperature ranges.

Unlike zero emission technologies such as wind turbines and solar panels, ORC plants can deliver baseload and flexible response power as long as the heat source is working. Therefore, these plants are optimally plannable and there is no need for complex grid stability studies. Applying an ORC plant to a newly built or existing system either increases the total output with the same amount of fuel, or generates the same output with less fuel. In both cases, the efficiency of the system increases significantly.

Standardized ORC solutions

Since 2001, Triogen has focused on developing and deploying a compact, modular, highly-efficient ORC power plant with up to 170 kW of power. With a fleet of over 40 ORCs in 11 European countries that has accumulated over 500,000 operating hours, the Netherlands-based company belongs amongst the most experienced players in the industry.

Henning von Barsewisch, CEO of Triogen, has a clear vision about the future potential of ORC technology. ‘Over the next two decades, Triogen will take ORC technology to the level of success that turbochargers showed over the last 40 years,’ he says, adding that the technology will ‘become a standard feature of combustion engines, reducing fuel consumption and emissions.’

Triogen offers the full range of services and assists heat source owners from the planning phase until the after-sales service. Since 2011, the Dutch company and its partner HeatConvert also offer attractive funding options to their customers. Among others, a full build-own-operate model and co-financing structures are available for heat source owners who are interested in realizing the economic benefits of the ORC, but would like to reserve financing capacity for other purposes. In addition, other financing options such as leasing and bank finance secured by export insurance are available.

No interference with an existing system

Another important feature of Triogenà‚´s ORC power plants is that they operate without affecting the heat source they are connected to. This is important because heat recovery is mostly an add-on rather than the core activity, so a fault or maintenance on the ORC cannot influence the performance of the core process. The major engine manufacturers have approved integrating a Triogen ORC system in the exhaust gas stream of their engines.

ORC plants also show an excellent part-load behaviour down to 35% of nominal power and they can rapidly change their output. The Triogen plants automatically adjust to heat changes, including startup and shutoff when needed.

Aside from the electricity produced, the Triogen ORCs can also be run in combined heat and power (CHP) mode. The heat available in the coolant can be used for drying or heating purposes. Using the heat in the cooling water can increase the total system efficiency up to 90%. Under normal operational circumstances the Triogen ORC produces coolant of up to 55à‚°C. When required, the new Triogen VARIO can increase the coolant water temperature to up to 90à‚°C.

Significant market volume

Today, Triogen is mainly active in the renewable energy markets in Europe. The company has gained extensive experience in combining its ORCs with various engines, fuelled by biogas, landfill gas, mine gas or diesel. In addition, the ORC technology, in combination with a furnace for the combustion of local, low-grade woody biomass, creates a sustainable CHP solution.

Operation and maintenance

Triogenà‚´s ORC plants have no gearbox, no intermediate oil or water loops and use direct evaporation. All rotating components are on one shaft, which means that the turbine drives the generator and the pump. The ORC operates fully automatically without any supervision. The housing is a hermetically sealed unit. For this reasons, the ORC plants do not require manned operation, are simple to operate and need very little maintenance efforts.

Also operational costs are low because no additional fuel is required. Only heat from the main system that would otherwise be wasted is required.

Modular design allows flexibility

Figure 2. The ORC as the prime mover in a biomass-based power generating system

If there is more heat than needed for a single 170 kW ORC, the units can be combined as modules. This maximizes efficiency and electrical power output, especially in plants with multiple heat sources that do not operate at full capacity all year round. Adopting a modular solution, with several small ORC units in parallel instead of a large power unit, allows for:

ࢀ¢ Higher efficiency at partial capacity;

ࢀ¢ Higher uptimes;

ࢀ¢ More flexible operation.

Case study: the Netherlands

In the case of gas engines, especially biogas, the additionally generated electricity qualifies for attractive feed-in tariffs or comparable incentives as part of many governments’ greenhouse gas emission reduction strategies. Using a Triogen ORC as a supplement to an engine can increase the plant’s power output by up to 10%.

One of Triogenà‚´s early customers in the area of biogas engines is the Kloosterman plant in the Netherlands, where corn is converted into green electricity. Fermenters produce biogas which is fed into two Jenbacher engines.

In 2009, Mr Kloosterman was looking for an efficient way to utilize the residual heat from the biogas engines and found an optimal solution with a Triogen ORC power plant. The exhaust heat from the two Jenbacher engines is redirected through the ORC power plant which transforms the heat into 155 kW of electricity. This efficient use of the residual heat from the engines increased the total electrical power output from 1672 kW to 1827 kW.

Because the ORC power plant is certified for unsupervised operation, Mr Kloosterman benefits from the additional, fuel-free electrical power output with virtually no effort or extra work on his or his staff’s part. The electricity, which is fed into the grid, is compensated with a feed-in premium according to the Dutch Electricity Law, the so-called ‘environmental quality of electricity production’ (MEP). The MEP provides a feed-in tariff of 16 €cents/kWh, resulting in an impressive payback period of less than three years at the Kloosterman project.

Mr Kloosterman himself is very pleased with his decision to upgrade his system with a Triogen ORC plant: ‘Triogen stated they can make 10% power on top of what our engines are already producing and they have proven it. The machine now runs smoothly along with the engines on approximately 99% full load on an annual basis.’ Since the beginning of operation, the ORC has produced 94% of the theoretically possible electricity, presuming that the heat from the engines was available at full capacity 24/7 every day since 2009. This very high capacity factor is in the same range as that of large conventional power plants and combustion engines.

Case study: Slovakia

The combination of an ORC with biomass combustion plants offers the opportunity to generate both electricity and heat from this renewable source. As in classical CHP plants, the high-temperature heat available from the combustion process is used for power, while the residual heat from the ORC cooling water is used for heating purposes. The term ‘solid biomass’ refers to a large variety of organic substances ranging from clean, new wood from the wood production industry such as saw mills to demolition wood, and residues from wood harvesting and other agricultural processes like rice harvesting, sugar cane, olive plants etc.

These latter materials generally have no other application, and are considered waste to be disposed of via combustion. The resulting CO2 emissions would have been created anyway, but they result from so-called short cycle carbon which is absorbed again in new crops. With this CO2-neutral process, usable electrical and thermal energy is produced in CHP mode, replacing fossil energy.

A good example of an efficient combination of heat and power is the Propopulo plant in Slovakia. At this biomass CHP site, the Triogen ORC power plant is situated in a sawmill that utilizes the residual biomass (in the form of sawdust) for combustion in an incinerator. The incinerator has a fuel input of 1110 kW using the untreated sawdust.

The Triogen power plant uses the heat from the combustion process to produce electricity and heat. The cooling water produced by this plant, which has a temperature of 75à‚°C, is used for drying kilns as well as space heating. The electricity produced is partly utilized by the sawmill itself, while surplus electricity is supplied to the central grid. Thus, the sawmill has significantly reduced the costs associated with buying grid power and generates additional revenues from the sale of surplus electricity.

‘The Triogen ORC secures power production that provides significant positive benefit to our company’s economic results,’ says Kristiàƒ¡n Slimàƒ¡k, Propopulo’s CEO. The plant owners plan to further maximize the use of its residual heat by adding an additional drying kiln powered by the remaining heat capacity of the ORC plant.

Great economics without feed-in tariffs

Even without the sale of electricity and the extra benefit of fixed feed-in tariffs, an investment in ORC plants provides interesting financial returns. This applies to projects where the generated electricity can be used to serve the system and/or used on-site, substituting for the purchase of expensive grid electricity. In Germany, for example, commercial customers often pay between €150 and €170 per MWh. At around €100/MWh, the electricity production costs of a Triogen ORC power plant are well below that level. A common Triogen ORC installation with a capacity of 170 kWe generates more than 1.3 GWh per year, the equivalent electricity consumption of almost 400 households. Assuming an average electricity purchase from the grid at €160/MWh results in cost savings of more than €200,000 per year.

Although the so-called EEG surcharge (an instrument of the German Renewable Energy Sources Act to fund incentives) also applies for self-consumption in Germany, the payback period for the ORC is often less than four years. Of course, paybacks or ROI estimates improve when the cost of the purchased electricity which it is replacing is increased.

Business case for new biogas plant

Incorporating heat recovery in the planning phase of new biogas plants allows for a unique business case. The capital cost per kW of an ORC is equivalent to the complete biogas plant. As the ORC can produce 8% additional power, the biogas plant can be scaled down by 8% and still provide the same electrical output. So the total investment remains unchanged, however the operating costs which are linked to intake, processing and disposal of digestible material are cut by 8%, resulting in substantially better returns. In addition, the effect of price increases for substrate, which have impacted biogas plants massively in the past, is reduced since less fuel is needed.

Below is an example of such a case:

Easy integration

The Triogen ORC modules arrive fully tested and pre-mounted at the customer’s sites. The dimensions of the Triogen ORCs are suitable to fit onto a standard truck to keep shipping costs low. Installation and commissioning mainly require some pipe works to the heat source and the original business process is interrupted for a limited number of hours. Due to the autonomous operation of the ORC unit, there is no further input requirement once it is running.

The more hours the heat source runs, the better for the ORC. A single year has 8760 hours and the Triogen ORC plants are built to operate for all of them.

Jos van Buijtenen is Founder of and Advisor to Triogen

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