(Note: Be forewarned this post reaches a stunning conclusion.)
Perceived through the constellation of undisciplined data processing algorithms deployed by the untrained Mind, it would appear that chilling and heating are but two sides of the energy coin- the yin and yang of thermal comfort. Do not be deceived, however. One is of a very Direct Nature, the other of a quite Indirect Nature, the relevance of which is to follow.
Consider a few rhetoricals…
What is the chilling equivalent of fire?
(The reader is encouraged to disregard the highly misleading content of certain beer commercials when reflecting upon this question.)
How many towns have a Volunteer Chilling Department to protect their inhabitants and property from the unexpected outbreak of bitterly cold, self-propagating, potentially out of control and in such cases terribly destructive phenomena?
One challenge in evaluating energy processes is understanding that although we can mathematically convert and express all forms of energy in the same units (e.g. kilowatt-hours or kWh, British Thermal Units or Btu’s, or, much to the chagrin of first-year engineering students throughout the English-speaking world, the quantity of slugs accelerated through a distance), they are in practical terms not all quite the same.
It is one thing to say a kWh of electricity is equivalent in energy content to an 807.3 lb sack of bricks located 0.621 miles above the ground, and another to figure out how to manipulate that sack of bricks properly so that by the time it hits the ground you’ve toasted 40 pieces of high grade spelt bread. And by toasted I mean charred.
Chilling, like the generation of electricity, requires a somewhat more convoluted energy conversion path than heating. The most straightforward means of generating chilled water are a) vapor compression refrigeration cycles, b) thermal absorption refrigeration cycles, c) evaporative cooling, and, when the outside air is cold enough, d) free cooling or direct sensible heat exchange with the outdoors. For the moment, we’ll focus on a) and b), because the last two options are impractical in hot, humid areas where the need for chilling is most intense, and because they also consume far less energy (and thus have less environmental impact)…
This first astoundingly blue chart shows the unit of chilling effect produced per unit of energy input. The electric drive chiller and heat pump are vapor compression cycle units, and the others are thermal absorption cycle units. No distinction is made between the type or quality of energy going into the systems.
This is not the whole story, however. It would seem that the vapor compression units are a much more strategic use of our energy input, but in order to run them we need high grade energy in the form of electricity (or rotary shaft power, also a high grade form of energy in the grand scheme of things). If an assumed 30% energy efficiency is included in the calculation to account for lost heat at central electric generating stations and grid transmission losses (based on the national average), the chart then changes a bit. See below, and note the Doppler shifted color scheme which subconsciously connotes an eroding confidence in our previous lucubrations.
The typical electric drive vapor compression cycle chiller still looks the best, but the gap has narrowed. Now, remember the other 60% of the energy input to electrical generation processes that is lost as “waste heat” because it is remote from the people and places that could use it? Let’s consider using that as our energy source. That could be used to run one of the absorption cycle chillers if it were a resource available on-site. There are also various and less-frequently-deployed heat engines such as packaged Organic Rankine Cycle units (vapor compression chillers with a different refrigerant, run backwards) that could be used to convert a fraction of that energy to electricity, and in turn run an electric drive chiller. Comparing these two scenarios we obtain the following (a stunning purple color scheme reflects compliments a stunning result):
The lower efficiency of the electric drive chiller reflects the diminishing efficiency in converting thermal energy into electrical energy as the temperature of the thermal source decreases. The Organic Rankine Cycle units just can’t convert low grade heat into electricity nearly as efficiently as a coal-fired power plant can convert high grade heat into electricity. The Btu’s are there, but we can’t convince nearly as many of them to become electricity once they’ve slid this far down the thermal value chain (which is temperature). Keep in mind that the low grade heat is generated as a byproduct for most forms of electric power generation serving the Grid. In other words, this tree is falling in the forest, whether you’re there to hear it or not. Just because you don’t have on-site electric generation pumping out waste heat faster than you can use it doesn’t mean its not happening somewhere else as a sidebar to the electricity you are consuming.
Now, in this last graph we’ll compare the energy input, carbon footprint, operating cost, and capital cost- ballpark figures only using Washington, D.C. energy costs for fuel and electricity- of two scenarios: a) using grid electricity to power an electric drive chiller (to make all options equal, we’ll make enough electricity to power the chiller plus an additional amount equal to the on-site generation), and b) using waste heat from on-site cogeneration to run a single-stage absorption chiller (assuming 80% of the waste heat is used and all of the electricity).
(Who needs color graphics, anyway?)
And now for the moral of the story: on-site cogeneration has the potential to provide chilling with low grade heat, while reducing your carbon footprint, energy cost, and your total energy input footprint. Although it is much less efficient than an electric driven vapor compression cycle chiller, it is able to utilize low grade heat as its energy source, and over 2/3 of the national energy input into electric generation processes is lost as heat to the atmosphere (or a large body of water).
And now for the real moral of the story: there are, according to the 2003 Energy Information Association Commercial Buildings Energy Consumption Survey, 71,568 million square feet of commercial buildings in the US. If each 200 square feet requires one ton (or 12,000 Btu/hr) of chilling effect on a hot day, the US needs about 400 million tons of chilling. Given the whole country doesn’t peak at the same time, this is without a doubt and unequivocably a high estimate (for space cooling).
Having said that, if we harness the waste heat from the electrical generation processes that go towards satisfying the very approximately 150 GW (yes, those are giga-watts) in electrical power those buildings consume, and use it for chilling, we could cover 15% – 20% (nearly 30% with double-effect absorption units) of the instantaneous chilling demand of the Commercial Fleet. We could cover more with thermal storage.
Lastly, if we actually were crazy enough to do this, (its not exactly practical on such a scale, I admit, but the trend is interesting), we’d also reduce the peak electrical demand at commercial buildings on hot days by roughly 30 – 40 GW, or nearly 25%. To put that in perspective, one coal fired plant generates about 1 -2 GW of electrical power output.
by Michael Mark, PE
