This post features concepts everyone can use to identify terrific savings opportunities, even if you did not take beloved calculus and thermodynamics. Let’s review a general hierarchy of typical energy efficiency families:
1. Shut it off
2. Slow it down, set it back (temperature, etc.)
3. Reduce waste
4. Retrofit or replace with efficient equipment
Those are all wonderful and probably capture over 98% of portfolio savings. Even newer programs like behavioral, retro-commissioning, and strategic energy management merely pursue these items. What’s left?
Smart design. I.e.,
5. Don’t be stupid
Or for the glass half full message, it would be “Be smart”, but this is the Rant, and that sort of language is simply unacceptable.
One thing to remember from this post is, when adding energy to a system, use the smallest increment possible.
There are two phenomena for this, and guess what – I will explain both of them!
First, don’t do more work than necessary. Second, the laws of thermodynamics favor small incremental steps to improve efficiency. Let’s cover both as follows.
Do the Minimum Necessary
Does this heading remind you of anyone – spouse? Coworker? Just wondering. We don’t want this from significant others or coworkers, but we do want it from energy consuming equipment.
Here are a few examples of what I am talking about:
Shown nearby is a simple refrigeration cycle like the one your refrigerator uses. Add a fan to blow air across the condenser, and we have a model for a central air conditioning unit like you probably have in your abode.
We have to ask ourselves, what do we want from this system? We want temperature and capacity, right? We need minus 10F to keep our ice cream frozen, and we need enough minus 10F to keep it hard.
To achieve this, we need to draw a low enough vacuum on the evaporator so the refrigerant boils at say, minus 20F. The major energy user in a refrigeration cycle is the compressor, shown. For a given design (I’ll explain that later), the compressor is stuck with that suction pressure.
What does “enough” minus 10F mean? On the refrigeration side, it means we need to move enough refrigerant (pounds) from liquid to vapor to keep the ice cream frozen. We need to move pounds and not necessarily the vaporous volume of refrigerant. We want to do this with the least compressor input necessary.
To do this, we can reduce the head pressure coming off the compressor. Tires are not the best example, but I will use them anyway. It’s easier to pump your bike tires to 50 psi compared to 110 psi, right? Same thing applies to a refrigeration cycle. How do we do this? By taking advantage of colder condensing conditions outdoors. Condensing pressure is directly related to condensing temperature. So when it’s cold outside, simply let controls reduce head pressure to minimize the compressor’s pressure lift and power input. Do the minimum necessary.
Incremental Baby Steps
My Master’s Degree thesis was funded by EPRI, and the focus was on optimization of Rankine cycle (steam, like those from coal, combined cycle natural gas, and nuclear) power plants. The basic Rankine power cycle, shown nearby, looks very similar to the refrigeration cycle, does it not? The difference is the flow is kind of reversed. It starts with heat transfer and ends with power leaving the system.
A real power plant diagram looks more like the next diagrammatic mess. I’m not going to explain it in detail (thank me). However, the HPHs and the LPHs shown at the bottom of the diagram are known as feed water heaters. Rather than running all the steam through the turbine, some is bled off at certain places to preheat feed water before it enters the boiler to produce steam again. Magically, this improves cycle efficiency. It adds or recycles heat in little steps to improve efficiency. My thesis was used to optimize the number and placement (at what pressures) to pull steam off the turbine for maximum cycle efficiency.
As with refrigeration compressors, the desired outcome of air compressors is pressure and mass of air compressed, NOT volume of air compressed. Compressed air flow is measured in standard cubic feet (volume), but designers need to think in terms of pounds. Here is why.
If you don’t remember the ideal gas law, the following should ring a bell dating back to junior high or high school:
In other words, pressure times volume divided by temperature is constant. Don’t let your head explode, just let me use an example. When air is compressed in a tire, the volume stays pretty much constant, as the tire does not expand like a balloon. As pressure increases, hopefully you can see that temperature also increases, such that the ratio stays constant. If you don’t believe it, grab your hand-pump compressor for your bike tires, pump madly to maximum tire pressure, and grab the bottom of the pump. It’s hot. That there is the ideal gas law in action.
Compressors move volume, but we really want mass flow. So what to do? Do it in stages. When refrigerant or air is compressed, it gets hot and is less dense. Less mass is moved as a result. Therefore, compress a little, cool it (it’s called an intercooler), compress more, cool it, and so on. This works for both refrigeration and compressed air.
We are out of room and time. I will cover “giant bounds”, maybe next week.