Sunday, March 24, 2013

Not So Classical Gas* - Energy Conversion Efficiency and Improvements in Natural Gas Technology

Last week I covered a ratio called capacity factor which some folks confuse with energy conversion efficiency. While the topic is still fresh, I thought I would cover energy efficiency this week to make sure there is a clear understanding of the two concepts. In my last post I noted that a great deal of energy technology is about converting energy from one form to another. For example, in a nuclear power operation we convert the nuclear energy holding the uranium nuclei together into heat which is then used to boil water to produce steam which drives a generator that produces electrical energy. The key to energy technology is to make these conversions as efficiently as possible.

The example I provided was my Prius versus a Maserati. I noted that the higher powered and larger engine of the Maserati could more rapidly convert the chemical energy in the gasoline into mechanical energy, i.e., rotation of the crankshaft, which is then converted into forward motion, or kinetic energy. However, I also noted that the Maserati does so less efficiently and that was cause for some smugness on my part.
 


  (Picture source: Maserati)

Let's be sure we understand the term conversion efficiency. Energy conversion efficiency is the ratio of useful energy produced to the input energy of the fuel used to drive an engine. It is typically calculated as a percentage: 
 
Energy Conversion Efficiency = (Output Energy/Input Energy) x 100
 
In automobile engines, energy conversion efficiencies are measures of how effectively the engine converts the chemical energy in gasoline into the mechanical energy of a turning crankshaft. The efficiencies are typically 25 to 35% for gasoline engines such as those found in a Maserati, 37% for my Prius and over 40% for turbocharged diesel engines. In fact, some turbocharged diesel engines can reach 47% conversion efficiencies. The rest of the energy is lost as waste heat. The useful mechanical energy that is produced is then used to overcome friction, turn the wheels and move me and the hunk of metal that constitutes my Prius from Concord to Manchester. Indeed, if we were to calculate the energy required to move one 200 lb man from Concord to Boston, we would determine, on that basis, that efficiencies are only of the order of 1%. The rest of the energy is lost as waste heat, waste energy during idling, overcoming friction, powering the devices in the car and moving the bulky metal can around me. That low overall efficiency is, for me, always pause for reflection.
 
The challenge with conversion efficiency is that one needs to be sure what one is comparing and recognize that there are many measures of efficiency. For example, when discussing automobile engines, we must not confuse fuel economy with energy conversion efficiency. Even though the fuel economy of a Prius is three times that of Maserati, it does not mean that the Maserati has a conversion efficiency one third of my Prius. The Prius owes most of its higher fuel economy to a lower vehicle weight plus its regenerative braking technology. In other words, the three-fold better fuel economy is more a vehicle issue than an engine issue.
 
While we are on the topic of engines, let's take a closer look at those extremely large engines in New Hampshire that are used to produce electricity. These engines can be coal-, oil-, wood- or natural gas-fired or even powered by wind or water. In this case we will measure efficiency by dividing the produced electrical energy by the input energy in the fuel which is used to drive the generator.
 
The table below shows the calculated aggregated conversion efficiencies for the various forms of electricity generation in New Hampshire. (Unlike last week, we have to resort to 2010 data as a full set of 2011 data is not yet available from the Energy Information Agency.) In this table, I have compared electricity output with the input energy consumption for each form of electricity production.
 

The conversion efficiencies are presented in the last column and are ranked from lowest to highest. The conversion efficiencies are generally low, and if we were to consider all the energy in NH produced from a single enormous generator – the Megarac 4500 from last week – the conversion efficiency for this device would only be 34%. The rest of the energy, 66%, is lost as waste heat.
 
Biomass has the lowest conversion efficiency, only 23%, partly because some of the energy is expended in driving off the water in the wood chips which can contain as much as 50% moisture. Coal-, nuclear- and oil-based fuels have conversion efficiencies in the low 30s. The conversion efficiencies for the State's hydroelectric operations are only 35%, which I found somewhat surprising: large hydroelectric operations are reported to have efficiencies of close to 90%, which means they can harness 90% of the energy in channeled water flow; even smaller operations are reported to operate with efficiencies of the order of 50% so the 35% figure for NH is a little puzzling. Modern three-blade wind turbines typically harvest about 40% of the available wind blowing over the turbine area so the 37% figure for wind is as expected. Natural gas, at 45%, has the highest conversion efficiency.
 
Natural gas is particularly intriguing as we are presently witnessing the large-scale switchover from coal- to natural gas-fired electricity generation. The driver for this switch has been low natural gas prices, but there has also has been a lot of research and development into gas fired turbines used for electricity generation which has significantly improved the conversion efficiencies of these devices. With modern gas-fired units, we have moved away from the classical way of generating electricity - burning fossil fuels to boil water to make steam to turn a generator which produces electricity. Instead, gas-fired generators are now of the gas turbine variety, where the turbine is propelled not by steam, but directly by the hot expanded gas that results from the combustion of natural gas. Moreover, because of new advanced turbine materials, we can run them at higher temperatures of operation which also improves engine efficiencies. With higher operating temperatures, we then have exit gases leaving at elevated temperatures. We are then able to harness these high temperature off-gases in a secondary operation to boil water to create steam to drive a secondary steam-powered generator. These units are known as combined cycle units and they can operate at very high efficiencies. A basic schematic of a combined cycle unit is shown in the figure below.
 
(Picture Source: Wikipedia)
 
The chart below, which is from an MIT report that reviewed advances in gas turbine technologies, shows how gas turbine technical advances have led to increased efficiencies for both simple(those without a secondary steam boiler) and combined cycle units (those with a secondary steam boiler). At the time of the report, units with 60% efficiencies were foreseen and today commercial units from both GE and Siemens are available that can achieve 60% or even slightly higher. These advances come as a result of years of turbine research for aircraft engines and energy generation as well as advances in materials that can withstand even higher temperatures.
 
 
It is these increased conversion efficiencies that are part of the attraction of natural gas. Not only does natural gas release less carbon dioxide per unit of input energy, but the energy conversion efficiencies are substantially greater than those of coal-burning operations: this further serves to reduce carbon dioxide emission per megawatt hour of electricity produced. In fact, per unit of electricity produced, carbon dioxide emissions from natural gas operations can be less than one half of that of equivalent coal operations. Over time, as equipment is improved, new investments are made and older, less efficient equipment is retired, I would expect the conversion efficiency of natural gas-based electricity generation in NH to creep up from the 45% noted in the table above. There clearly is a lot we can do to improve the existing efficiency of natural gas combustion in New Hampshire.
 
Returning to the conversion efficiency table above, we note that conversion efficiencies for technologies other than natural gas are surprisingly low. Technological advances over time should improve these. However, breakthrough advances, like those we have seen for natural gas, are unlikely. Instead, we will see small incremental improvements over time and, in my mind, continual improvement in efficiencies should be part of the operating philosophy of every electrical generator and should perhaps even be part of the permit to operate. Small incremental improvements in conversion efficiency can result in large bottom-line benefits for both generating companies and for the planet, as this will reduce the amount of carbon dioxide released per megawatt hour of energy produced.
 
But, instead of waiting for breakthrough energy efficiency technologies, there is something we could do right away. We should be focusing on that 66% of waste heat. We should be investigating applications where we harness that unused heat and find ways to distribute it to the community, such as you would find in district heating applications that are in common use in many of the northern European countries. If we did this, then we would certainly be getting away from the classical way of doing things - simply burning fossil fuels to produce steam to turn turbines which only generate electricity and a lot of wasted heat.
 
Hopefully this week I have left you with an appreciation of how low our energy conversion efficiencies are for electricity generation and that there are opportunities for improvement. You should also have a good understanding of the difference between energy conversion efficiency and capacity factor. Remember, energy conversion efficiency is the ratio of useful energy output to the input energy and capacity factor is the ratio of a generator's actual output compared to what theoretically could be achieved if the generator could be run 24 hours, 365 days per year.
 
Until next time, remember to turn off the lights when you leave the room.
 
Mike Mooiman
Franklin Pierce University

mooimanm@franklinpierce.edu
3/25/13

(*Classical Gas is the name of a guitar instrumental tune, written and performed by Mason Williams in 1968 who, at the time, was a writer on the Smothers Brothers Show. For an instrumental, it was a big hit, reaching #2 on the charts. Forty five years later I still think it has a lot of appeal. Take a listen to the acoustic version without all the orchestral filigree.)

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2 comments:

  1. The challenge with conversion efficiency is that one needs to be sure what one is comparing and recognize that there are many measures of efficiency. After long time i have seen very technical post. I really enjoyed it.

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