Schoolgen Sister Schools Cluster Day at Greenhithe Primary School.

Weeks of planning came to fruition.  Maggie, Katie and myself had worked hard to get the first “sister schools” day off the ground. The day went very well and the two activities I had designed seemed to work and be enjoyed by almost all the students. According to the evaluation forms they particularly enjoyed the turbine making activity :-)! It even made the North Shore Times with a photo of a girl (aged 10) admiring her freshly minted turbine. The children were from 12 schools in the area and ranged from Years 5 to 8. They were all very well behaved and keen to learn (such a refreshing change!). The format was 5 activities of half an hour interspersed with 3 presentations, morning tea and lunch. the time frame was quite tight for the turbines but was just enough. The kids were able to make a turbine with me and Caroline’s assistance and got 5 minutes teaching on Pelton wheel turbines and how they generate electricity by moving a magnet past a conducting coil.

I was lucky to have the props of my F&P smart-drive still attached to the plastic tub from the washing machine, an Ecoinnovation Pelton wheel micro-hydro generator from work which uses a rewired smart-drive as its generator.

The kids loved seeing the sparks when one of them turned the rotor and the other brushed the wires and were able to easily see the magnets and coils when the rotor was screwed off. They were fascinated that you could power your house with one of these. I left them with the idea of continuing on their enquiry by building a housing for their turbine and attaching a motor/generator via a gear, again seeing the example I had made was empowering for them.

Using the excel spreadsheet to explore simple heat conduction.

I have started working on creating my own simulation of heat conduction. I hope to create an interactive flash program which the user can play with to visually and numerically see how conduction occurs in a 3 dimensional object, initially a cube. To get to grips with the basic physics of it I setup an excel spreadsheet and took advantage of the little used but powerful feature of excel which enables automatic and repetitive calculations or iteration. Iteration is repeating or looping a function over and over again, feeding the output of one calculation back into itself. This allows one to see how things change over time, each loop of iteration is a step in time.

The spreadsheet takes an array of cells whose initial temperatures are defined. Each cell looks at the temperature of its neighbours and if they are lower, leaks away some heat to them according to the heat conduction equation. This then decreases the temperature of the cell by an amount that is determined by the specific heat capacity of the substance. Each cell has a formula that references itself, which excel generally avoids, as it is often a mistake in a normal spreadsheet giving you a ‘circular reference’ error. This is avoided by allowing iteration. To access the iteration feature, select Tools>Options>Calculation: Tick the iteration box and set the number of iterations. Press F9 to execute the iteration.

Thermal Modeling of an Auckland House using SunREL

Using SunREL* to model a hypothetical house to see the effect of windows/glazing on solar gain, the amount of incoming solar energy into the house which leads to heating of the internal zones. To model a building you must specify all aspects of the building- its dimensions, the orientation of the walls, the positions of the windows, slopes of the roof, the makeup of the walls, roof and floor, their thermal properties (resistance, conductance, specific heat capacity and so on). Also the location of the building in terms of latitude and longitude to determine the position of the sun throughout the year (or whatever period you intend to model the house for).  A climate record is also needed for the area; since I am modeling a house in Auckland, the program must reference a weather file representing an average of Auckland’s climate (which I presume consists of temperatures and sunshine hours, but over how many years and to what level of detail I am not sure at present).

 

Heating, ventilation and cooling (HVAC) systems may also be indirectly specified by defining a temperature inside the house below/above which it is not allowed to fall/rise. I set the minimum temperature to 20^oC so that when the air temperature fell below this set-point the heating would come on. No air-conditioning was specified as this should not be necessary in an efficient NZ home! Cooling through forced ventilation when the temperature moved above 21^oC was specified.

 

Once the basic building was setup and running bug-free (this took me a while to get the first time I tried!), I specified standard double glazing (IGU) in all the windows, with 30% of each wall (facing north, east, south and west respectively) as window and then ran the file. The output of the run file showed me all sorts of data such as the amount of solar radiation falling on the windows through the year, and how much of that was actually transferred into the building, giving the so-called “solar gain” for the month/year (measured in Giga-Joules), I could then see how that affected the mean and maximum temperatures in the house throughout the year, what size heating/ventilation plant would be necessary (measured in KW) to maintain the building within its specified set-points and so on. Note- does Sunrel specify heating capacity for a modern heat-pump or a conventional resistive element heater?? The output also told me how much energy was lost through the windows, walls and floor, and into the attic space through the ceiling.

 

Once I had the basic file operating properly I just altered the dimensions of the north (sun) facing window so that I had a range from 10% to 90% glazing area for the north wall (other windows stayed constant at 30% glazing). Once this was done I went back to 30% glazing for all the windows and created a horizontal overhang to shade the north wall. I then varied the length of the overhang between 0 and 3 metres and was able to see how this affected the energy entering and leaving the house for an average Auckland year. In this scenario I was also interested in how the overhang affected the solar gain in the shortest days of the year around the Winter Solstice in June, and the longest days of the year occurring around the Summer Solstice in December. Since the sun is much lower in the sky in winter you would expect an overhang to have little effect in winter, but a larger one in summer when, if the overhang is properly sized it should shade the interior from the midday sun.

 

* The program is not particularly easy to work with given that it is based on a very old program written using the FORTRAN language in the 1960s to run on mainframe computers. All building data is input in multiple small windows looking like mini excel spreadsheets. The building file is then run and the output generated must be opened using excel as a tab delimited text file. For ease of reading I would then save this as a text file and open it up in notepad, scroll through the pages of output data and transfer the relevant data into another excel spreadsheet which I had setup, I could then easily graph the data to see how changing the amount of north facing window

 

I am now looking forward to learning how to use a much more modern and sophisticated program called TAS which shows a 3D architect like drawing of the building.

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Heat Conduction

The following is my summary of relevant parts of a wikipedia article on Heat Conduction and other bits and pieces of information.

Conduction is one of the three ways in which heat can be transferred, the other ways being convection and radiation. Conduction can simply be thought of as heat transfer by “touch”, and only can occur in matter (solids, liquids and gases). Heat energy always flows from areas of higher to lower temperature, it therefore follows a negative temperature gradient. When the temperature has been equalised then a state of thermal equilibirum has been reached and no further heat transfer occurs. Heat transfer in solids is caused by vibration of neighbouring molecules and the movement of free electrons. In liquids and gases, molecules are in random motion and they transfer energy to each other by colliding, and when moving from areas of high to lower concentration (diffusion).

 

Heat transfer in metals is dominated by free electrons transferring heat energy. Because of this, thermal conductivity of metals is related to electrical conductivity. Metals are generally very good conductors, and usually non-metals are poor, the remarkable exception to this is diamond which has a thermal conductivity of up to 5 times greater than silver. This property can be used by gemologists to differentiate real from fake diamonds. In non-metals, phonons (quantised lattice vibrations in crystals) have a larger role in thermal conductivity. See phonon article if further information is desired.

 

Heat transfer in gases is generally low if convection is minimised, air for example is a good insulator when confined in small spaces, such as within animal fur like wool/feathers, or insulation foams used in building and other industries. The inert gas argon (1.8 kg/m³) is denser than air (1.2 kg/m³) and has a lower thermal conductivity, which is the reason it is used in double glazing (k of argon = 0.018 W/m.oC, k of air = 0.025). Lighter gases generally have higher thermal conductivites than heavier gases. Hydrogen is therefore used as a “coolant” in the steam driven rotors of large power stations, such as at Huntly thermal power station (k of hydrogen = 0.18).

 

The law of heat conduction is also called Fourier’s Law after the great French mathematician and physicist Joseph Fourier. It states that the flow rate of heat (measured in Watts) through a material depends on the temperature difference or gradient that exists between two surfaces (measured in degrees Celsius or Kelvin, per metre of thickness), and the thermal conductivity of the material. The thermal conductivity (symbol= k) can be thought of as the heat flow rate through a given square meter of surface area (ie. a flux), for each unit of temperature gradient. The units for thermal conductivity are therefore W/m² per K/m which equates to units of W/m.K or W.m^-1 K^-1.

 

Material	Thermal Conductivity(W/m.K)
Aerogels	0.01
Argon	0.018
Air	0.025
Building insulation	0.04
Water	0.6
Glass	1.1
Concrete	1.3
Sandstone	2.4
Steel	25- 50
Aluminium (pure)	220
Copper	380
Diamond	up to 2300 !

 

Thermal conductivity should not be confused with thermal conductance which is the heat flow through a surface of given area (A) and thickness (L), which equals kA/L and has units of W/K.

eg. A 50 m² concrete wall, of 150mm thickness with one degree celsius temperature will have thermal conductance (or heat flow) of: 1.3 x 50/0.15 = 430 W.

 

Generally, in the building industry one works with thermal resistance or “R-values”, which is the resistance to heat flow of a building component of given material and thickness. The R-value is calculated by the thickness of the wall/roof etc divided by the thermal conductivity. This then allows the various R-values to be added to give one overall R-value for a building element such as a wall, which is typically made of several layers of materials. The heat flow through the wall/roof is then given by the area divided by the thermal resistance ie. Heat Flow = Area/Resistance.

 

If the walls in a building are made up of different sections having differing construction and/or materials the heat flow calculation must be done for each section and added together to find the total heat flow. From this an average weighted R-value may be found R-weighted = total area / total heat flow.