Costing the UnitValues in the Cost Analysis were obtained using Seider’s Process Design Principles: Capital Cost Estimation. To estimate the cost of the condenser and evaporator, the principal equipment cost was found (CP) from the following equation found in Seider’s for cylindrical process vessels.
Where L is the length, or height of the vessel in meters, and D is the diameter in meters. Using the values we calculated earlier, the condenser had a height of 3.67 meters with a diameter of 0.61 meters, while the evaporator had a diameter of 0.67 meters and a height of 3.64 meters. Overall, because the dimensions of each the condenser and evaporator are very similar, their costs are also in the same range. For the heat exchanger, CP was found from the following equation, also found in Seider.
Where A is the area of heat exchange in m2, which was 60.3 m2. Once the principal equipment cost was found, we could then find the bare module cost. The bare module cost includes direct and indirect project expenses. These include materials used for installation, installation costs, labor, insurance, and taxes. The bare module cost for the condenser and evaporator was calculated from the following equation, where L and D are the same as above, FM is the materials factor, and P is the operating pressure in bars.
The materials factor changes depending upon what kind of materials we select. Cost analysis comparing carbon steel, stainless steel, and titanium were all completed to compare costs of materials and to see how much of a difference it made. The values of FM for the different materials can be seen in the Table 10.
Table 10: Values of materials factors
Once the bare module cost was found, a number of other costs must be found, including: cost of site preparation and service facilities, the cost of contingencies, cost of land, royalties and start-up, and working capital. Once all these are calculated the total capital investment is found. The total capital investment was found for situations of different materials being used for the evaporator and condenser, which can be found on the next page. It was decided that carbon steel could be used for the condenser, but stainless steel must be used for the evaporator to avoid corrosion. Also taken into account was the costs of miscellaneous materials (solar cell, nozzles, and pumps) of $10,000. Once this was all taken into account, the total cost of our entire unit would be $540,607.66.
Table 11: Individual costs
Once we knew the cost of the entire unit, we assumed that it would last 25 years. Considering this, we took the number of liters per day produced (5673 liters/day) and assumed that production would only occur six months out of the year, and receive the insolation (solar energy) for January throughout the year, rather than increasing for all other months (worst case scenario).
We then took the total cost of investment for our desalination unit, and divided it by the number of liters we would be producing, and the final cost comes out at $0.02 per liter of water.
Assuming the average person uses 250 liters of water per day, that becomes $5/day, which in turn becomes $150/month. More research must be done to determine how this unit could be more cost effective.
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