Earthquake Resistant and Energy Efficient Homes - PAKSBAB

After the devastating earthquake of 2005 which destroyed nearly half a million rural homes in Pakistan, there was an urgent need to build earthquake resistant homes. Thus came into being PAKSBAB which is the short form of Pakistan Straw Bale and Appropriate Buildings. So far PAKSBAB, from its limited resources, has build 27 homes in northern parts of Pakistan. These homes have been built using indigenous resources and by training the local people in construction of straw bale houses.

A typical straw bale house has an area of 576 square feet and costs $3000 on average. This  turns out to be $5.2/square feet which is less than half the cost of brick and mortar houses. A typical home comprises of two rooms, a verandah and a kitchen and requires around 1200 hours of labour. So 6 people working for 8 hours daily can construct a straw bale house in 25 days!

The main advantages of straw bale houses over brick and mortar houses are highlighted below.

1. Energy efficiency, since straw is a good insulator

2. Non toxic products are used (light straw, clay and wood)

3. Cheap materials are used resulting in a cost that is half that of a regular house

4. Resistant to earthquakes

Energy Efficient House

Tightly packed walls and a gravel weighted foundation creates better weather-proofing

 

Energy Efficient House

Twice as energy efficient as a conventional house, straw bale makes for environmentally friendly earthquake-proof homes

Energy Efficient House

Clay-plaster reinforced, a fabricated straw bale house costs half the expenses of modern building for every square foot

Straw bales required for the construction of these energy efficient and earthquake resilient homes are built using manually operated farm jacks and locally manufactured compression moulds. Furthermore the local industry is being encouraged to supply straw bales and other materials required for these projects. Additional appropriate building methods that PAKSBAB is promoting include passive solar, rainwater catchment, solar lamps, high-efficiency cooking and heating, and the use of natural building materials such as light straw clay, wattle and daub, and cob.

Solar Cell Temperature and Efficiency

It is a common misconception that higher the temperature higher is the output of the solar cell. This is not true as the efficiency of a solar cell decreases with increase in temperature and lower efficiency results in lower output power. So in fact a bright sunny day, with sun rays perpendicular to the solar panel and a cool weather is the ideal combination for higher performance of a solar panel.

Let us now look at this in a bit more detail. There are two basic reasons for decrease in efficiency due to increase in temperature.  One is the decrease in the band gap energy (Eg) and the other is the decrease in open circuit voltage (Voc) with increase in temperature. The relationship between band gap energy and and temperature is quite straightforward and is given as.

 E_{g} = E_{g}(0)+\genfrac{}{}{1}{0}{\alpha T^2}{T+\beta}

Band Gap Parameters

 

One might argue that the decrease in band gap would allow for more carriers to be transferred to the conduction band and yield a higher output power. However this is not true as the output power is the product of current and voltage and a lower voltage would reduce the power. In fact there is an ideal range of band gap which produces the maximum energy. Going too high or too low would not yield the optimum results.

Next we turn out attention to the open circuit voltage Voc. The relationship between temperature and open circuit voltage is not that straightforward. At first it might seem the open circuit voltage increases with increase in temperature as shown in the expression below.

 V_{oc} = \genfrac{}{}{1}{0}{k_{B} T}{e} ln \biggl[1+\genfrac{}{}{1}{0}{J_{L}}{J_{s}}\biggr]

But in reality this is not the case. Increase in temperature results in increase in intrinsic carrier concentration n which in turn results in higher reverse bias saturation current Js. There is a squared relationship here so increase in intrinsic carrier concentration would cause a very large increase in reverse bias saturation current. And as is evident from the above formula this causes a decrease in open circuit voltage. This is also shown in the figure below.

Voc and Temperature

So to conclude a bright sunny morning in winter might not be the worst time to produce some solar energy (provided you have got your solar panel tilt right 🙂 ).