Organic Rankine cycle (ORC) systems have been used for decades to generate power. The generators work much like a steam engine; however, instead of water, ORC systems use a fluid with a much lower boiling point than water. As a result, an ORC generator can use low-grade heat (<150°C) to produce electricity.
Since their initial development, improvements in turbine efficiencies and heat transfer materials have been incorporated into ORC systems, improving their performance. But the technology remains largely under-utilized, tapping only a fraction of the potential market due to the upfront costs of the systems and relatively high cost of the power produced.
Higher Efficiency Means Increased Affordability
Sponsored by the DOE’s Geothermal Technologies Office, PNNL “rewrote the book” on the operation of ORC systems in an effort to make a transformational advance in the technology and its use within the geothermal industry. The PNNL-developed system—the Harmonic Adsorption Recuperative Power System (HARP)—uses a novel approach that eliminates the need for the evaporator, high pressure pump, and condenser in ORC systems.
Conventional ORC systems work by transferring heat to a working fluid until it vaporizes at a constant pressure. The high-pressure vapor is passed through a turbine, or other engine, that produces electricity. The vapor is then condensed back to a liquid and recycled using an electric powered pump. The HARP cycle scraps all those components and substitutes a patented multi-bed heat engine architecture and a specially formulated metal organic framework (MOF) sorbent to drive the engine.
In the HARP system, the working fluid vapor is adsorbed into the nanostructured pores of the MOF sorbent, resulting in near liquid phase density. This process takes the place of condensing the working fluid vapor back to a liquid state. Instead, the MOF sorbent pores are packed to capacity with the working fluid. When heat is subsequently applied to the MOF, the working fluid vapor is released from the MOF pores, and tremendous pressure is generated. This process takes the place of evaporating the working fluid back to a vapor state. In the HARP system, the working fluid remains in a vapor state the entire time; the only difference is that the working fluid vapor attaches, detaches, and reattaches into the MOF pores. A set of four heat exchangers packed with the MOF work in tandem, adsorbing and desorbing the working fluid in a “harmonic” cycle that swings the pressure and temperature every one to two minutes. The multibed heat exchanger system thus becomes a thermal compressor that powers the HARP system.
Efficient, but How Efficient?
The elimination of the ancillary components in the ORC allows the HARP system to generate 40 percent more power from the same heat source. Even better is the cost of electricity when using HARP: only $0.05/kWh. Compare that to $0.15-0.20/kWh of a conventional ORC system. A 10 kW unit—big enough to power about eight average U.S. households—can be paired with many different sources of low grade heat, including waste heat from diesel generators, solar arrays, produced water at oil and gas sites, or shallow depth geothermal sites. Scaling up to 1 MW or even larger systems is straightforward due to the simple design PNNL staff developed for the thermal compressor components, which is readily manufacturable at scale.
HARP has the potential to transform power production from low grade heat sources that are uneconomic to exploit today. No heat is "waste heat" with the HARP system. Staff are putting the finishing touches on the first HARP system that should be generating power by early next year.