Reducing the Impact of Energy Transmission and the application of Expanding Gas Power Transformation (EGPT)
The debate around how energy is harvested, produced, transmitted and used has been mainstream for more than 3 decades. As we head to the close of the first decade of the 21st Century it is fair to say that we have seen some shifts in the way we produce and use energy, with a large focus on renewables for generation and more energy efficient processes for the end use.
However, there has been little change to the key transmission and distribution infrastructure for the delivery of much of this energy, something we will still be dependant upon well into this century, in fact most of the drive for change will see even greater expansion of this infrastructure. So what are we doing to ensure that it operates in the most efficient manner? Are we applying our engineering skills to Green our primary energy infrastructure?
One of the key energy transmission systems in question is used for the delivery of natural gas across the planet. While fossil fuels in general are considered a non-sustainable option in the longer term, all forecasts show them playing a large part in the energy mix for many decades. NG is considered the most efficient and environmentally friendly of the group and with extensive transmission and distribution systems already in place lends itself to being adopted to the transmission of Bio-Gas, as well as Hydrogen injection generated by electrolysis from excess renewable electricity.
Like all transmission systems, there are parasitic losses. In the case of NG this happens mainly during compression and decompression, or Let-Down losses. Compression is self explanatory and the use of ever increasing efficiency in compressor technology sees regular reductions in the energy load. The Let-Down side of the process is a little less evident and losses occur as a result of the cooling effect of gas expansion, better known as the Joules Thompson effect. To explain, whenever we reduce the pressure of compressed gas through a throttling device, the temperature plummets. Not a problem if you are not worried about thermal stress and condensation, however the materials used for NG pipelines and the fact that all NG while predominately Methane, also contains a cocktail of potentially corrosive elements, compel us to ensure that we keep the gas temperature around 4 degC or greater. But as engineers we can deal with this.
The funs starts when we look at how to offset this. The universally adopted method is to burn some of the gas and heat the gas by passing it over hot water coils. Simple effective engineering, but it pushes carbon into the atmosphere from combustion processes that range from 90% efficiency down to a miserable 15%. Some would argue that it is only a small percentage of the overall energy throughput, but it is still a considerable amount when looked at in the round. To give an example, if we were to remove this combustion process from our gas infrastructure we would be saving the equivalent of the total amount of electricity currently generated on the planet by Concentrated Solar Power, or if we only removed 50% we would save the same output as the Three Gorges Dam, or 22GW. Quite a prize to go after we think. But how can we achieve this; reduce the overall gas consumption, eliminate any CO2 emissions from Let-Down and thus increase the energy and environmental efficiency of the process.
The answer actually lies in the exploitation of the very cooling effect of gas depressurisation and the use of the very emission we are trying to eliminate; CO2.
I was fortunate enough enough to work with two bright people; one a Refrigeration Engineer, the other a well respected Energy Physicist. While examining ways of future-proofing the exploitation of heat pumps to produce hot water we came across a process known as the Transcritical CO2 Cycle (TCC). In summary it is a refrigeration or heat rejection process that uses CO2 above the critical phase and thus at high pressure, to achieve extremely high (up to 130 degC) rejection temperatures. The net result is the ability to generate hot water at high temperatures. During discussions around possible applications a problem previously posed by a gas utility was examined anew and we realised that this cycle was a perfect match for the problems posed by NG expansion. EGPT was born and we set about finding a suitable partner for its exploitation, the search resulting in a partnership with GE. Roll on 10 short years and the very first exploitation is currently being commissioned in Bologna, Italy and due to go into service by end of February 2019.
So what is Expanding Gas Power Transformation?
It is in its simplest description a combustion-free electrical energy generation process, using gas expansion and free heat as the energy source. Instead of using a throttling valve to drop the pressure in the line, we pass it through a Turbo Expander and expand and cool it even more aggressively. This results in the thermodynamic energy being converted via the TE shaft into electrical energy. The trick in the process is the use of the TCC heat pump, a perfect match to the heat demands of the TE gas expansion process. The only other thing we need is a heat source; this can be waste heat, geothermal, or in the case or the site in Bologna the ambient air. We literally deploy a bank of TCC units sitting in the air and cool the atmosphere while generating 80 degC water for the gas heating process. The TCC heat pump is operated at a Co-efficient of Performance (COP) of 3.5 to 4.0, meaning that for every kW of electricity generated by the TE we get a 3.5 to 4.0 kW heat input into the process. The result is that in a typical 300kW TE installation we eliminate all need for combustion to offset the parasitic heating demand and at peak we export up to 75kW of surplus green electricity to the grid.
The next steps are to collect the numbers and get the utility world on board. The potential to offset the need for another 2 Three Gorges Dams by better efficiency in the NG transmission and distribution system is an attractive energy goal with the added benefit of the elimination of a unwanted CO2 discharge into the atmosphere, while using a benign refrigerant medium in the form of CO2, not to mention other obvious environmental gains.
Smart but simple solution, we think. Having just returned from a trip to see it in the flesh, we are are proud of what Irish engineering can deliver on a global level.
For further information on the process see www.egpt.ie.
About James Byrne:
James Byrne is one the founders and a director of the Sirus Group. He began his career in the family engineering business and went on to study Plumbing and HVAC technology in Bolton Street College (now TUD). He has spent a number of years in the early 80s working as a consultant designing HVAC systems and during this period elected to specialise in the application of Building Management Systems (BMS).
His interest in BMS, HVAC and energy has led him to establish a number of companies over the past 30 years dealing with many aspects of building technology and energy efficiency. He has led a number of projects dealing with the sustainability of buildings and energy processes and was behind the refurbishment project know as The Well. This project demonstrates how an inefficient 80s industrial building can be re-imagined as a healthy and energy efficient office environment. He also engages in energy projects examining the application of basic physics to modern energy problems, with a view to producing sustainable solutions.