The following is the assessment of week 7:
A company has asked for proposal to develop a cordless, wall mounted power supply, giving six volts, presumably for security cameras, smoke detectors, and other equipment. The new power supply should replace four D-cells, which last about six months. The company states that the benchmark gives 0.55 mA continuous current and 151.2 seconds, 800 mA burst per hour. The new power supply should last twice as long, if it needs to be replaced, the procedure should only take a couple of minutes. The company seeking this technology has focused on three ideas:
- A better battery
- A small fuel cell
- A battery supplemented by a solar cell running off
building lights, which recharges the battery
Our task is to choose the best
candidate that comes closest to meeting the company's criteria.
The following are the options of
power sources with their properties:
|
Voltage (V)
|
Energy/Volume
(W hr/L)
|
Charge time
|
Cycles
|
Cost($)
|
|
|
D cells
|
1.5
|
60
|
2 h
|
1000
|
15
|
|
Lead acids
|
2
|
40
|
8 h
|
1000
|
10
|
|
Lithium
|
3.6
|
100
|
1 h
|
1000
|
50
|
|
Hydrogen fuel cell
|
0.8
|
10
|
2 min
|
Large
|
500
|
|
Methanol fuel cell
|
0.5
|
80
|
2 min
|
Uncertain
|
500
|
We have come up with the following table to better compare all of the candidates with the criteria given by the company
|
Selection Criteria
|
W
|
D cells
(Benchmark)
|
Lead acids
|
Lithium
|
Hydrogen fuel cells
|
Methanol fuel cells
|
|||||
|
Lifespan
|
0.3
|
5
|
1.5
|
5
|
1.5
|
5
|
1.5
|
8
|
2.4
|
4
|
1.2
|
|
Charge time
|
0.3
|
5
|
1.5
|
2
|
0.6
|
6
|
1.8
|
8
|
2.4
|
8
|
2.4
|
|
Voltage
|
0.15
|
5
|
0.75
|
6
|
0.9
|
8
|
1.2
|
3
|
0.45
|
2
|
0.3
|
|
Energy Density (Size)
|
0.15
|
5
|
0.75
|
4
|
0.6
|
8
|
1.2
|
2
|
0.3
|
6
|
0.9
|
|
Cost
|
0.1
|
5
|
0.5
|
6
|
0.6
|
3
|
0.3
|
1
|
0.1
|
1
|
0.1
|
|
Total
|
5
|
4.2
|
6.0
|
5.65
|
4.9
|
||||||
More weightage are give to 'Lifespan' as the company specifically mentioned that the new power supply should last twice as long. Weightage for 'cost' are the lowest as its importance is less compare to other criteria for this particular company
The properties of Energy Density (W hr/L) is used to rank the size of the batteries and fuel cells as higher energy density indicates smaller batteries or fuel cells. Lifespan of the power source are compared from the number of charge cycles for each of them. Higher charge cycles means the batteries or the fuel cell are able to charge more number of times before they reach the end of their life.
The results from our selection table shows that lithium is the winner from all the potential candidates. Although it doesn't last twice as long and takes more than 2 min to recharge (1 hour). Its small size with high energy density and its high voltage makes up for what it's lack and came up as superior to other power source candidates.
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