The intermittency challenge
Published by Jessica Casey,
Editor
Energy Global,
The climate emergency has made the transition towards a low-carbon world not only essential but also urgent. While a lot of progress has been made in the technology underpinning renewable energy supplies (such as solar and wind) in recent decades, its characteristic intermittency continues to present a challenge. Weather conditions and daily solar cycles create variability in energy production, and this results in a discrepancy between supply and demand. As a result, developing high-tech solutions for short, medium, and long-term energy storage is crucial.
While batteries are proving to be an effective solution for short-term energy stor-age (less than four hours), their high initial costs limit their economic viability for medium to long-term storage. In contrast, mechanical and thermo-mechanical energy storage systems use less expensive storage mediums, making them a more cost-effective solution for longer durations. Historically, pumped hydro has been the pre-dominant medium to long-duration energy-storage solution, but its dependence on specific geographic locations limits broader adoption. With less geographic constraints, thermo-mechanical systems become an important alternative. At the heart of most thermo-mechanical energy-storage technologies is power-generating turbomachinery, which is crucial in the system’s efficiency, reliability, and adaptability.
This article provides an overview of the success of turboexpanders in organic Rankine cycles (ORCs). It then goes on to show that the expert technological knowledge and experience from this field can be directly transferred to solar and thermal-hydro energy-storage facilities. And as shown in a pilot study, this provides a method to better deal with the variability inherent in renewable-energy sources.
Turboexpanders and ORCs
Turbomachines are not a new technology, and turboexpanders have been an important element in the energy industry for many decades. The term ‘turboexpander’, or simply expander, originates from turbines specifically used in industrial refrigeration. Cryogenic radial inflow turboexpanders were developed in the late 1930s for use in air separation, and by the 1960s, their effective refrigeration and power recovery capabilities led to their widespread use in natural gas processing, petrochemical production, and refineries. The tried and tested features that enabled turboexpanders to excel in these applications were later recognised as beneficial in other hydrocarbon-utilising applications, such as ORCs.
Turboexpanders have long played a central role within ORCs, a process used in the transformation of low to medium-grade heat into electricity. ORCs function in a similar way to traditional steam cycles but use an organic working fluid, such as butane or pentane, which have significantly lower boiling points than water. Historically, ORCs have proved valuable in geothermal power production and waste heat recovery, where the working fluid can be effectively matched to the heat source, enabling efficient energy conversion. It has been shown that machinery selection within these cycles can further benefit the process. For instance, turboexpanders equipped with variable inlet guide vanes (IGVs) provide flexibility for use where ambient temperature fluctuations may alter operating pressures in an ORC. The ability to run efficiently at a wide range of operation allows for additional energy recovery when ORC condensing temperatures vary, such as with daily or seasonal weather patterns.
Thermal-hydro energy storage
More recently, ORCs equipped with flexible turboexpanders have begun to be utilised in energy storage systems. In contrast to the steady operation of baseload geothermal plants, energy-storage systems require rapid start-ups, frequent start-stop cycles, and fast shutdowns because of the dynamic characteristic of the energy networks. These elements need careful consideration at the turboexpander design stage to maintain performance and longevity.
A case study highlighting the success of such a system can be seen in a pilot plant that opened in August 2023, in Carwarp, Victoria, in the southeast of Australia. Named RayGen Power Plant Carwarp (RPPC), it combines solar and thermal-hydro energy-storage facilities while utilising ORC technology with Atlas Copco Gas and Process turboexpanders.
RPPC integrates a combination of proven technologies in a novel way. Figure 1 shows how the solar collector and thermal-hydro energy-storage function together as a unified solar power plant. A field of smart mirrors concentrates sunlight onto photovoltaic solar modules in a tower-mounted receiver. This produces 1 MW of electricity for every 2 MW of heat, with a combined energy conversion efficiency of around 90%. The heat is removed by cooling water, which is then stored in an insulated pit at near boiling temperature. At the same time, electricity sourced from the photovoltaic system (or the grid) powers a chiller to produce cold energy stored in a separate water pit. The stored energy can be then converted back to electricity using an ORC at a chosen time, producing stable electricity even when the sun is not shining.
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Read the article online at: https://www.energyglobal.com/special-reports/16072024/the-intermittency-challenge/
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