Posts Tagged ‘air conditioning’

A Thermo Problem – Need a “Rule of Thumb”…

July 4, 2012

Happy 4th!

I have a question for you thermodynamics wizards – I need a rough answer – a “rule of thumb”, so don’t get carried away with precision and nuance considerations. Here is the problem:

What is the “degrees per hour economy” to set my AC thermostat up when I leave the house? If I will be gone for 10 hours, and I set my thermostat on my AC up to save electricity when I leave the house, and then set it back again when I return, how many degrees up must I set the thermostat to save money on my electricity bill?

The intuitive answer is “…any period of time with a higher thermostat setting will save electricity.”

But, wait a minute…  The AC runs at a 100% duty cycle to cool the house from the higher “away” setting to the cooler “occupied” setting.  There is a cost just to transition from the “away” steady state to the “occupied” steady state temperatures that may exceed the cost saved maintaining a warmer interior temperature for a period of time.

There is a thermal gradient at the perimeter of the house – exterior walls, foundation and roof that transfers energy from outside to inside during the summer. A house’s thermodynamic behavior also has a characteristic analogous to inertia – everything in the house absorbs thermal energy, so you must do more than cool the air – you also must cool a portion of the physical mass of the structure to transition between steady states or maintain a state. The significant consideration to frame the problem and then solve it is therefore to specify differential between the inside temperature and outside temperature for the “occupied” and the “away” states, and specify the time in the “away” state. The time in the away state also includes the time for the structure to “relax” – the zero-cost time when the house warms to the “away” state.

So, you “thermo-heads” out there – I know of two of you reading – what is the energy use relationship that helps me decide the minimum away time to bother with and the critical thermostat setting necessary to save energy? There ought to be a simple rule of thumb…


I have a rule of thumb from Oncor:

Set the thermostat up for your two story home in summer 2-3 degrees – just 2 or 3 degrees… for an unoccupied period of 8 to 10 hrs. The rationale:
– The home warms about one degree per hour and cools two degrees per hour (deg F).
– Warm air rises (from the first floor to the second) – more than 2 or 3 degrees F and the upstairs AC compressor will attempt to cool the first floor, too – you can stress it to failure.
– the refrigerator also may be over-stressed to cool when the ambient exterior (the home interior) is more than 40 deg F refrigerator / 60 deg F freezer warmer.
– wood floors can dry and warp and crack.

Just 2 or 3 degrees… I am banking on 4 or 5…

10 Crazy Ideas – one is likely to be BIG!

June 12, 2011

I have ten crazy ideas. One of these will be a big life-changing or work-changing idea – maybe just for me, but perhaps for many, many people. Here they are:

  1. A spam filter for my phone. If your number is “Unavailable” or “Telemarketer” or “Insufficient Data”, my phone will never ring, and we won’t talk! Optionally, if your company is unknown to me, we won’t talk, either. And optionally, if you are on my “black list”, we won’t talk – ever – we do the black list today, but that’s the extent of a spam filter for the phone.
  2. Streaming media that follows me wherever I go. When I leave the kitchen where CNN is on the TV and step into the breakfast room, CNN will automatically stream to the family room TV when I get there. When I leave my house and get into my car, CNN will stream to my car through my car speakers. When I get to my home office upstairs, CNN will stream to my desktop there. CNN – or whatever I am watching or listening to – will stream to the media system of my choice wherever I go – the best media system wherever I am. Automatically. Without missing a beat.
  3. A universal media license for my own content. If I own a DVD, I should be able to stream it from any source for a data fee if there is one without paying another license fee. Apple is doing this to some degree with iTunes that will stream your music library to any iTunes-compatible device you own without uploading your media to Apple. Sweet. They beat me to this one… We’ll see how this works.
  4. Free education. I am certain that information creation and discovery will happen at an ever increasing pace in the future – hold on to your seat, because this will be a wild ride. How do you manage this? Keep current in your career? Learn new ideas that shape your life and the lives of those around you? Ideas that could be key to a long and happy life? You consume education, that’s how. The pace of change will become so fast that comfortable survival will demand continuous education of everyone. If education costs money, a great many who’s priorities or finances don’t support a learning mission will be left behind. Unless education is free for the taking, that is, and then anyone who wants to learns things of importance – for their entire lives. On our present course, though, a smaller and smaller portion of the population will be able to afford an ongoing education for their entire lives.
  5. A shower and an aerobic exercise machine like a StairMaster on commercial airplanes that cross an ocean. Charge extra for that? Sure, why not. I’d pay.
  6. Virtual movie actors that members of the audience become. What? If you are one of the first in line for a movie, you choose a character in the movie to direct – to become – to act. In a sense, you become one of the characters. You make decisions about the actions of that character within certain bounds, and so do a number of other people in the audience for other characters. Every running of the movie would be a unique experience.
  7. An instant, real-time language translator. Not really so crazy. People are working on this – maybe my good friends at Google… When I phone my good friend in Japan and talk (in English), he should hear MY voice speaking Japanese. And I should hear HIS voice speaking English. Effortlessly.
  8. Local environmental control in the home. Huh? When I am in bed at night, why control the temperature of the rest of the house to the same temperature like we do today? When I am in my home-office, why control the temperature of the bedroom to the same temp? There is so much opportunity to make improvements in energy usage patterns in the home… Someday, we’ll catch on to this. Environmental control should follow people. We do this with lights in some homes today, but anything more extensive demands new construction techniques to isolate spaces. control airflow, and facilitate control from more locations than just one or two thermostats.
  9. A new focus on mental health. Treat the homeless and criminals for their mental illnesses that presently destroy their lives (and the lives of others around them) and diminish their future value in society. Everyone has potential – everyone has unmet potential. Between 15% and 25% of the population suffers from untreated mental illness that impacts their lives, limits what they do with their lives, and often adds a burden to society where these people live. That’s a lot of lost productivity, expense, and a lot of misery we simply allow to persist by being passive with regard to these ill people.
  10. Flush cancer cells from the body. One day, nanomachines will scour the body entangling themselves in cancer cells they have an affinity for and eliminating these cells from the body. Everyone is walking around with cancer cells in their body. Rather than killing these cells and many more in the body to fight cancer after a few of these cells become active and tumor-producing, just “round ’em up and head ’em out!”
  11. A bonus, since several ideas aren’t so crazy or so far “out there”. A motion-based battery charger that you wear to charge a cellphone or an iPod while you walk. How far to fully charge a cellphone? A couple of miles? I could get rid of my “wall warts” and never need to plug in again… Do that for my iPad, too. If I can carry it, it should recharge when I move about.

Well, there are eleven  crazy ideas. Time for smart people to “git crackin'” on some of these. And for a few dedicated people to promote these ideas – oh, that would be me… Write back with a crazy idea of your own!