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Last updated: 23 January 2019

New Heat Pump System

Hang-Gliding for Research Purposes

A British aeronautical engineer, Jonathan Howes, has come up with an ingenious idea while hang-gliding. His plan is to launch a meteorologically powered engine to fly him between thermal currents, the columns of rising air which glider pilots use to gain altitude. After reaching the desired height, he would use a thermal’s updraught to turn a small turbine which caught some of the thermal energy in order to store it. Afterwards, when in level flight, the pitch of the turbine blades would change and it would become a propeller driven by its own stored energy. The truly fascinating matter is that the captured energy isn’t electricity, but actually consists of heat and cold in two small reservoirs. Mr Howes has already started working on a proper-sized mechanism as part of his search for a cheap and reliable alternative in storing electricity on the grid, a development which would transform the use of renewable energy.

Obtaining power from renewable sources such as the wind and sun are intermittent. Although this disadvantage, costs have started to come down and the efficiency to increase, thus renewables are being used more and more. At the moment, there are a number of technologies being improved so that they can store energy on the grid, such as flow batteries that can acquire energy in liquids and discharge fast. Huge flywheels and supercapacitors, are also researched. Howes has the belief that his invention can beat them all.

Isentropic is the company Howes co-founded with two partners, James Macnaghten and Mark Wagner, in order to build this new system. The system revolves around a heat pump, the company developing several prototypes of what the trio call pumped-heat electricity storage (PHES). They are close to completing a presentation unit with an output of 1.5 megawatts for further testing at their factory near Fareham, on Britain’s south coast.

Isentropic's Idea

PHES subsists of two silos, each one filled with gravel. These silos are connected at top and bottom through a series of pipes filled with argon, an inert gas. When the electricity hits a surplus level, it starts running the motor which operates a compressor to squeeze the argon. Compressing this gas makes it heat up, in this case argon reaching 500 degrees celsius. As it’s pumped through the first silo, the heat is being transported to the gravel. The cooler gas which emerges is then made to expand, thus lowering its temperature close to -150 degrees celsius. The difference that occurs between the two silos is used to generate electricity by reversing the process. The gas heated by the hot silo discharges into the cold one, driving the cylinders of what had been the pump in order to turn what had been the motor as a generator.

With the existing systems, there is always some loss in energy storage. Isentropic has stated that PHES is able to deposit and discharge electricity with a “round-trip” efficiency of 72-80%. This compares with 74% for pumped-hydro, the way 99% of grid-scale storage takes place at the moment. Hydro-pumps use their surplus electricity to pump water up to a mountain lake and then let it flow down again to operate a hydroelectric plant if and when required. This is an efficient system, but would require a mountain nearby to make it work.

PHES’s efficiency is developed from the design of the pump/engine, due to the fact that it engages a number of advanced hydrodynamic principles, which includes a novel sliding valve built inside the head of the pistons. The system is low-cost, compact and has a long service life, Wagner has been recorded saying.


Isentropic has calculated the cost of storage of a PHES system, based on its capital cost which is amortised over 25 years and its running costs, is £32 for a megawatt hour. Because the price of hydro-pumps varies a lot, installations are different, but it usually reaches the value of £42 for a megawatt hour. Although Howes doesn’t have a plan to scale this system down for his hang-glider, he is thinking about developing a smaller version designed for domestic use. Thus, house owners would be able to store surplus electricity from solar panels or a wind turbines, granting people a possibility of energy independence.