Careful analysis suggests that, for most parts of the US, buildings using a high efficiency, gas-fired trigeneration system can emit less carbon dioxide than those using conventional energy supplies and a solar photovoltaic installation – writes Wes Livingston of Power Partners.
The United States is blessed with an abundance of natural gas, coal, and solar energy. It is also the world’s second highest producer of carbon dioxide (CO2) and a desire to reduce these emissions has prompted federal and state governments to partially fund solar power systems while discouraging coal consumption. Natural gas, however, has been promoted by some and discouraged by others. This paper will show that a facility using a properly sized trigeneration system burning natural gas will likely emit 25% less CO2 per year than a traditional building using supplemental solar power.
|A rooftop based trigeneration system in Atlanta, Georgia, in the US – the adsorption chiller (right) provides 40 tonnes of refrigeration to the building|
First of all, solar cells obviously emit no CO2 while operating. Instead, the emissions arise from the grid when solar power is not available. Coal-fired power plants are still the most common power source at most utilities.1 These power plants are located away from urban areas. So the utilities are not able to harness the abundant exhaust energy created by combustion. Instead, those precious BTUs are simply released to the atmosphere. Typically, at least half the heat energy from the fuel must be wasted since it cannot be cost-effectively converted to electrical energy.
To reduce emissions from electricity, many people turn to on-site photovoltaic (PV) panels to produce electricity from sunlight. A solar PV array sized for the peak electrical load of the building can only offset the electricity from the grid for some fraction of the day. Due to clouds, night-time darkness, and shading, most of the electricity for a building must still come from the grid. Also, any heating or cooling requirements for the building must be produced by ancillary equipment such as natural gas fired boilers or electro-mechanical chillers consuming significant amounts of electricity.
|Figure 1. CO2 savings for solar PV and trigeneration compared to cost.|
In contrast, an on-site trigeneration system burning natural gas simultaneously produces electricity, heating, and cooling at very high efficiency. An engine burns natural gas to spin a generator to make electricity at 30% to 45% efficiency. The waste heat from engine coolant water and captured exhaust heat is used to generate hot water or steam for free. In the summertime, the hot water can be used to drive a thermal chiller to provide cooling for very little additional energy input. By utilizing a domestic fuel with high hydrogen and low carbon content (CH4) and also utilizing the low grade excess heat to heat and cool the building, the trigeneration system emits lower rates of CO2 than solar PV with grid backup. Also, the capital costs for a trigeneration system are far less than for a comparably sized solar PV array with separate heating and cooling systems.
According to the US Department of Energy, power plants in the US emitted 2.5 billion tons (2.27 billion tonnes) of CO2 in the year 2009.2 That is more than ten times the combined weight of the American population, or roughly 900 kg per American dumped into the atmosphere. And that is the backup power supply for solar PV. For 2009, the DOE states that 3.95 trillion kWh were produced in the US.3 Therefore, we can easily calculate that the American utility grid emitted 0.58 kg of CO2 per kWh in 2009. This is a notable improvement from the year 2000 when 0.65 kg of CO2 per kWh were emitted. The decreased CO2 emissions are due to increased natural gas use, decreased coal use, more efficient power plants, and increased use of carbon-free renewable fuels.
|Figure 2. Comparison of electrical and thermal outputs from an equally sized trigeneration system and solar PV system. Note the significance of daily run-hours.|
Coal plants in the US emit about 0.96 kg of CO2 per kWh while large natural gas turbines produce 0.59 kg of CO2 per kWh.4,5 Even after hydroelectric, nuclear, wind and solar generation are included in the national grid calculation, the average American electric utility emits 0.58 kg of CO2 into the atmosphere for each kWh of electricity produced. This is due to the fact that 44% of the electricity generated in the United States comes from coal, and coal emits 74% of the CO2 emissions.3,4 On-site trigeneration systems significantly reduce emissions by burning hydrogen-rich natural gas and then harnessing waste exhaust energy to provide free heating and cooling.
Solar PV, however, can only generate electricity. The other main concerns with solar power are the lack of availability (low capacity factor) and the high cost. Although the sun itself gives off energy continuously, a surprisingly small percentage reaches our rooftops consistently due to clouds and Earth’s own shadow. Even in the southern California desert, the sunniest location in the US, only about seven full solar hours of sunshine are available during an average day.6 Similarly, Atlanta, Georgia only receives about 4.5 full hours of solar power per day on average. Therefore, a 100 kW array in Georgia would only produce on average 450 kWh per day. This is in contrast to a 100 kW tri-generation system that would produce 2400 kWh per day plus 3000 kWh of free heat. This issue of capacity factor is very important for emissions calculations and return on investment. Atlanta’s solar availability is only 4.5 full hours per 24 hour day. That works out to 18.8%. However, a trigeneration system can run continuously except for maintenance and therefore has a capacity factor that approaches 100%.
The sunniest locations in the world have a solar capacity factor of just 25%. Therefore, a solar PV array would effectively produce its full rated capacity for just six hours per day, every day of the year. A nominal 1000 kW solar PV system in Arizona would produce 2,190,000 kWh per year (1000 kW x 8760 hours x 25%). In a hospital with a constant load, the remaining 6,570,000 kWh would need to be produced by the grid. (Note that this calculation does not include any shading losses or the 23% loss that NREL estimates for a normal solar PV system caused by soiling and DC to AC conversion.7 This analysis is truly the best case for PV.)
|Figure 3. Graphical representation of solar PV resources in the US, courtesy of NREL|
Since the solar PV system must rely on grid power for supplemental electricity most of the time, the net result is a system that emits 0.43 kg of CO2 per kWh of electricity consumed even with a solar PV array sized for the full capacity of the instantaneous peak load of 1000 kW. To save more energy, the PV array would need to be larger than the electrical load of the building. The extra energy would then need to be sold back to the grid or stored on site in batteries. Either option would require significantly more financial investment.
Instead of solar PV, you might choose a high efficiency on-site trigeneration system with waste heat capture for heating and cooling. Assuming 33% electrical efficiency for a microturbine, CO2 emissions of 0.55 kg per kWh would be emitted solely for electricity production. In Arizona, that’s not an improvement compared to solar PV, but we have not considered the free waste heat that would be used to offset the heating or cooling loads.
By capturing the waste heat from the microturbine, a traditional boiler can be eliminated. Assuming a standard 82% efficient boiler, an additional 0.22 kg of CO2 per thermal kW are eliminated by the trigeneration system. This extra heat can be created for free with zero emissions. That is the benefit of on-site power generation. It creates less CO2 than solar PV with grid backup. If the heat is taken as a credit, the trigeneration system emits a net 0.33 kg of CO2 per electrical kWh. Or taken as a complete system, the trigeneration system in heating mode will emit 0.55 kg of CO2 per kWh while the equally sized PV array with boiler would emit 0.75 kg of CO2 per kWh. This is a 26% savings for the trigen system compared to the best case for solar PV. In other locations outside the desert, the analyses make PV look far worse due to lower solar capacity factors.
|Figure 4. CO2 emissions in heating mode (winter) for grid power with boiler heat, grid power with supplement solar PV and boiler heat, and on-site power production with heat recovery.|
For example, in New Jersey, the solar PV capacity factor is only 18%. A typical building with a PV electric array producing 1000 kW of electricity with grid backup plus 1421 kW of thermal output from an 82% efficient boiler will emit 0.79 kg of CO2 per kWh of electricity produced over the course of an average day. But an on-site microturbine trigeneration system would only emit 0.55 kg of CO2 per kWh in combined power and heating mode. That is a 30% reduction.
Even if the heat is not used, the solar PV system will emit 0.48 kg per kWh while a reciprocating engine generator will emit 0.43 kg per kWh. In New Jersey, the natural gas generator will emit less CO2 than PV with grid backup even if the heat is not captured at all.
This has political consequences as well. Tax dollars are paying for solar PV panels all over New Jersey that only produce electricity 18% of the time.8 Trigeneration systems could do the job 24/7 with fewer emissions at much less than half the capital cost.
What about summertime in a data centre?
A trigeneration system sized at 1000 kW of electricity output can provide enough heat to drive a 250 ton (3,000,000 BTU/h) cooling system for free by using an adsorption or absorption chiller. The same electrical and cooling capacities using PV would require a 1000 kW array plus a 250 ton chiller driven by grid power. Assuming an electrical efficiency for the chiller of 0.65 kW per ton, an additional 163 kW of grid power would be required even while the PV array was active. At night, the building would require the full 1163 kW of power entirely from the grid to match the output of the 1000 kW trigeneration system. Therefore, in Arizona with a 25% solar capacity factor, the overall net emissions rate for the solar PV system plus chiller would be 0.53 kg of CO2 per kWh of electricity produced. In New Jersey, with the lower capacity factor, the PV system would emit 0.57 kg of CO2 per kWh.
|Figure 5. Installed costs of equally sized trigeneration and solar PV systems compared to the cost of each system divided by its capacity factor (percent of time it produces energy each day).|
With trigeneration, however, the full electrical output of the generator could be used for needs on site while the chilled water would be a free by-product. Therefore, using a 1000 kW reciprocating engine/generator delivering 250 tons of cooling output, we would find emissions of 0.48 kg of CO2 per kW of electricity produced in either Arizona or New Jersey. That would result in emissions savings compared to solar PV of 8.6% and 15.9%, respectively.
A high efficiency trigeneration system would not only cut the emissions significantly. It would also reduce the capital cost. PV arrays require about $5 to $6 per watt installed, just for the solar system. In contrast, a complete trigeneration system including the heat recovery system, chiller, pump package, and installation would cost about half that. Although the trigeneration system would continue to consume fuel while the solar PV system would not, the low cost electricity produced by the trigen system would dilute the savings from a PV system. According to our calculations, the facility that installs a solar PV system would need at least 30 years to recover the investment (at 0% interest) and would emit more CO2 over that time compared to trigen. In fact, according to NREL’s own calculator, a solar PV system in Atlanta, Georgia would take 60 years to pay back without government incentives.9 That is far longer than the projected life of the solar cells.
What if I don’t need heating or cooling?
As stated above, in locations such as New Jersey, a high efficiency generator burning natural gas only being used to produce electricity with no heat capture will still emit less CO2 than a PV array with grid backup. To gain even higher efficiency, an Organic Rankine Cycle heat recovery package could be used to boost the electrical output and reduce emissions by 10% or more. Again, the cost is significantly lower than a PV system. And the net result is a system with lower CO2 emissions than PV with grid backup. If America’s goals are to reduce CO2 emissions, use a domestic fuel, and spend the least amount of money possible, a serious consideration of natural gas trigeneration systems should be made. Site conditions should be evaluated for the benefits of using trigeneration’s free waste energy for heating, cooling, and additional electricity production.
|Figure 6. Energy costs from each source and the years required for a straight payback.|
Let me close by stating that I am certainly in favour of harnessing solar power and other renewable energy sources. But with limited money to spend, the most cost-effective solution for CO2 reduction should be implemented. In many cases, trigeneration will unquestionably be the better solution. Perhaps future advances in solar technology will result in mass produced, inexpensive solar cells, solar mounts, inverters, and energy storage. But we cannot change the weather. Solar PV will always be handcuffed by limited run-hours and limited solar intensity. For most parts of the US, a trigeneration system will emit significantly less CO2 than a solar PV project due to the limited operating hours of solar PV and the continued emissions from the grid.
- 2_report/CO2report.html” target=”_blank”>https://eia.doe.gov/cneaf/electricity/page/CO2_report/CO2report.html
Wes Livingston is a design engineer at Power Partners, a manufacturer of adsorption chillers and factory-built trigeneration systems. E-mail: firstname.lastname@example.org
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