About neil

building simulation-part1


Typical House Energy Flow (click to enlarge) courtesy of Arron Acosta and Cy Hoadley-Kilbourne, MIT

If you’re contemplating an energy upgrade to your house or commercial building, perhaps someone has mentioned the concept of a building simulation, or doing an energy model.  Such a process has been part of the commercial HVAC world for some time now.
As the name suggests, a building simulation involves the use of a computer to simulate the building’s energy performance.  The goal of performing a simulation is not only to predict how much energy a building will use over the course of a year, but also to compare alterrnative designs. We can also look at some measures of comfort within the building.

With all this data, it’s clear that a simulation can be very powerful for helping make decisions. As you might guess, authorities that regulate energy compliance as well as those that give out rebates want to see this kind of analysis done to make sure they are spending our money wisely.

The ‘sankey’ diagram above illustrates the complexity of the energy flows into and out of a typical house.  Based upon a description of the building input by the user, the building simulation software calculates all the energy flows into and out of the building. Not only does the software model the building envelope, but it models the interactions of people, infiltration, and even exotic things like trombe walls. After doing all this, the software has to predict how the building equipment will perform. Everything from furnace efficiency, daylighting controls, through daylighting controls is simulated on a 24hour/365day basis.

The pay-off is not to predict the exact energy use of any one particular house, but instead to look at alternate ways of building, different equipment, and even alternate control strategies.

Over the next blog postings, I’ll keep you in the loop on our progress.

Executive Summary:

Financial analyst is to Spreadsheet AS   Engineer is to Energy Modelling

slug and inertia

Not the slug in question

A little background information will may make this post funny for you…

The system of measurement that we in the US use is sometimes called Imperial units (Imperial as in British Empire), but it goes down hill from there. As an example of this,  US ounces are not and have never been the same as the Canadian (Imperial) ounce.  The fluid measurement system then goes on to make cups (8 ounces), pints (16 ounces, but 20 in an Imperial pints), gallons (128 ounces, or 160 in an Imperial gallons).  It’s all commonplace to us locals, but maybe, just maybe this helps explain our national distrust of numbers (apparently mortgage rate resets are particularly vexing).   Hmm.. maybe we could work out a system in which all units are related by multiples of 10, there are no multiple versions of measurements (ounce, pint, gallon, bushel, barrel, acre-foot, acre-foot ??). How about that metric system.

So, on to the slug. As the metric system has the kilogram as its unit of mass, we have the pound. Oops. Pounds are units of weight.

What is our unit of mass?  Yes, you guessed it. The slug.  Yes, and this is what makes high school physics so exciting. Knowing that your buddy has a mass of 5 slugs.

So here are the factoids:

1 Slug on earth gravity weighs 32.2 Pounds.

1 Kilogram on earth gravity weights 9.81 Newtons.

Why does the US have so much inertia when it comes to changing to the logical metric system ?  I don’t know, but physical inertia derives from mass, and our mass unit is a slug.

References:

Wikipedia

http://www.diracdelta.co.uk/science/source/s/l/slug/source.html

NASA criticised for sticking to imperial units

town names

Wisdom, Montana 1942

(Photo by John Vachon. Prints and Photographs Division, Library of Congress)

I don’t know why this town did not grow and thrive. They were clearly years ahead of their time.  Putting your town’s name for air travellers to see is, in my opinion, the most important thing a town government can do.  How many times have you looked out the window of an airplane,  noting a beautiful vista, only to never find out what city you were flying over ?

Trains and coal strange bedfellows

We love railroads. Who can’t love their efficiency. BUT… They like coal. They really like coal. According to this report 44% of railroad car traffic is coal. And coal is dirty. It’s dirty in terms of carbon emissions. It’s dirty (but can be cleaned) in terms of particulate emissions. It’s not all that wonderful in terms of mercury dispersal either.

So, which way do you think railroads will lobby (they are corporate “citizens” after all).  Will they throw their weight towards laws which reward efficiency => win for rail transportation. Or will they throw their weight behind more coal burning => still good for railroads.?

Transportation energy use

Since energy is a topic of conversation in our office, and I have been doing some bicycle commuting, it suddenly became important to have a table of transportation energy use.

Mode kW-h/km kcal/mile Miles/gallon kW-h/mile beer-pints/mile
Bicycle 0.0361 50 629 0.058 0.23
747 0.2521 350 90 0.407 1.59
Electric Car 0.1550 215 146 0.250 0.98
SUV 1.5128 2098 15 2.440 9.54
Train 0.0820 114 277 0.132 0.52

Conclusions:

  • Bicycle wins the efficiency contest.
  • Electric car could win the efficiency contest if you fit 5+ people in your car.
  • Escalating that war, the tandem would probably be the next weapon of choice.
  • Trains are pretty good (no surprise), and perhaps could get better with regenerative braking.

Assumptions:

  • Bicycle uses 50 kcal/mile
  • 747 90 mpg (FAA figure is lower,but…)
  • Electric car: .25 kwh/mile (from Tesla  if you trust software ‘geniuses’  posing as a car company, or if you trust the old fashioned car company GM, their ‘Volt’ appears to be .34 kW-h/mile)
  • <rant> Tesla presents data like a software vendor (no surprise).  Their claim a 70amp @220 volt charger, running for 3.5 hours charges the battery fully (capacity listed as 55kw-h). Energy = efficiency (always less than 1.0) x 70 x 220 x 3.5 = 53.9. Clearly credibility = 1/ efficiency </rant>
  • SUV 15 mpg
  • Train (swiss) .082kwh/passenger-mile
  • Pint of beer has 220 kcal
  • kcal (kilocalories) are the kind we eat (aka Calories with a big ‘C’)  Yep, our system of measurement is THAT stupid. Small calories x1000 = Calories =  kilocalories.

Conversion Notes:

  • It’s assumed that all liquid fuels have the same energy content as gasoline: 36.6 kwh/ gallon.
  • Liquid fuels have crappy energy conversion, unlike electricity which can be 100% efficient.

Solar Loves Whole House Fans

There are a couple of trends that will affect solar photovoltaic installations:

  • Newer electrical rate structures encourage conservation (tiered rates)
  • ‘Energy Security’ is a powerful motivator
  • Photovoltaic rebates require house efficiency check-ups

We think  solar energy gathering is great. The idea of  getting electricity essentially for  free energy has certainly captivated our imaginations. Even though the cost of photovoltaic panels is dropping very quickly, the investment is still considerable.  Our crystal ball tells us that electrical energy is very, very versatile and will be in demand as electrical cars come on stream and coal fired electrical power plants start to show their ugly side. (coal burning)

As the demand for clean electrical energy increases, its value will certainly not go down.   In fact, if the validity of predicting cheap energy such as the famous line is any guide “too cheap to meter”,then we should get prepared for high prices.

By combining whole house fans solar contractors and  homeowners  can:

  • Decrease the initial investment in PV panels
  • AND/ OR
  • Increase the  Return on Investment for the entire system