| Organic Rankine Cycle
to be vaporised. The LNG is regasified, going from cryogenic supercritical conditions (-160 °C, 80 bara) to around 0°C, a temperature deemed suitable for injection into a national gas grid.
The condensed organic fluid is then pumped up to the required pressure prior and then re-enters the evaporator in a closed cycle configuration, minimising any potential leakage of the organic fluid.
The organic fluid is chosen to optimise turbine performance as well as the entire cycle, as well as taking into consideration EU environmental regulations, which forbid the use of fluids with global warming potential (GWP) greater than 150. The advantages of LNG regasification employing
Above: 24 MWe geothermal power plant in Turkey employing EXERGY ORC technology
example, geothermal brine and industrial waste heat recovery applications – operating between say around 200°C and ambient – ORC technology, with suitable choice of working fluid, can also be applied at much lower temperatures, to extract power via cold energy utilisation, making use of the cold energy contained in liquefied natural gas with seawater as the heat source.
Exergy is developing its ORC offering for LNG applications and is introducing innovations aimed at increasing efficiency relative to other ORC technologies being used for cold energy recovery in LNG regasification applications. During the LNG production phase, natural gas is compressed and chilled to liquid state to achieve volume reduction and thus facilitate transportation, which is done in insulated cryogenic ship tanks at atmospheric pressure. To liquefy natural gas it is cooled to -160 °C at 1 atm. Approximately, 500 kWh per ton of LNG is needed for LNG compression and refrigeration, which makes cold energy exploitation interesting from a thermodynamics point of view. In fact, the cold energy available in LNG – taking it from -160 °C at 1 atm to 0°C at 80 bara – is approximately 740 kJ/kg of LNG.
Currently, only about 1% of the power generation potential available from cold energy utilisation is being exploited. It has been estimated that over 12 GWe could be produced, and, with increasing concerns about energy supply security, exacerbated by the invasion of Ukraine, interest in LNG is growing. Historically, Japan was the first LNG importer worldwide, and remains the largest, with China forecast to take the lead by 2023, with Chinese LNG imports projected to reach 128 bcm/y by 2025 thanks to the continuing expansion of the country’s regasification capacity. Meanwhile, India is expected to overtake Korea as third biggest LNG importer worldwide.
UK, Spain, France and Italy are the major European LNG importers, with increasing attention put on flexible floating storage and regasification units (FSRUs), like OLT Livorno, a terminal moored 20 km offshore Livorno coast with 4 MTPA peak send-out rate. Germany is rapidly pushing ahead with
several infrastructure projects for the import of LNG to reduce dependence on Russian natural gas. Germany’s first LNG terminal is under construction at the port town of Wilhelmshaven, and contracts for four FSRUs have been signed. After being transported, LNG is pumped and regasified, exploiting the heat input provided by a thermal source.
Common technologies used for LNG regasification mainly comprise open-rack vaporisers (seawater as thermal source) and submerged combustion vaporisers (natural gas as thermal source). Alternative approaches include intermediate fluid vaporisers and ambient air vaporisers. None of these regasification technologies make use of the substantial quantities of cold energy available in liquefied natural gas. The Organic Rankine Cycle, however, provides an effective means of harnessing the cold energy contained in LNG.
Referring to the schematic process flow diagram below, seawater is used as the thermal source to vaporise an organic fluid (typically propane), which is then expanded in a turbine connected to a generator for electricity production. The organic fluid, which is still in vapour phase, is condensed, exploiting as cooling source the LNG
the Organic Rankine Cycle include the following: ● Unlike an open-rack vaporiser, electricity is produced thanks to the expansion of the organic fluid in the turbine. The electricity produced can be employed to power balance of plant and auxiliaries (such as LNG pump, seawater pump, boil-off gas compressors, etc.) or supplied to the national grid at medium voltage if there is a surplus.
● Compared to a submerged combustion vaporiser, no fossil fuel is burned as part of the LNG regasification process since only seawater is employed in the evaporator. As well as generating power, the ORC system achieves significant primary energy savings, while also avoiding the emissions resulting from combustion.
On the downside there are potential issues related to the low temperatures and pressures in the turbine and condenser. But these can be minimised by selecting an optimal organic working fluid and employing materials already well established in the cryogenic sector.
Growth market
Given that LNG global trade and LNG demand is continuously growing year by year, there will be increasing opportunities for the deployment of Organic Rankine Cycle systems for cold energy exploitation, paving the way for continuing efficiency improvements at LNG regasification terminals worldwide and contributing to lower operating costs.
Sea reservoir Generator Sea water 25°C Evaporator Condenser
0°C
LNG storage tank
Sea water 15°C
ORC
LNG
Above: Basic process scheme for an LNG regasification plant employing an Organic Rankine Cycle. Temperatures and pressures shown here are examples. The actual figures will depend on specific project details
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