Background

Parras de la Fuente

Parras de la Fuente is a desert oasis town of about 44,000 residents located in the south of the Mexican state of Coahuila. Along with textile manufacturing, tourism is one of the main industries in Parras. Tourism has become increasingly emphasized after Parras's designation as the "primer pueblo mágico del Norte de México." Parras is warm in the summer, and cooler in the winter, but temperatures rarely fall below freezing, snow falls once every several years.

Elevation

1,505 m

Longitude

102°11'W

Latitude

25o30’ N

2005 History

In 2005, Señora Rosa Guadalepe Vinelna welcomed Parras program participants to install a solar hot water system at her home, Zaragoza #1, near La fabrica mundial de la mezclilla in Parras. The system was constructed in 2005 by Alyssa Grassi (aag16 «at» humboldt.edu - why does this email address look different?) with help from plumber Mario Cardeñas (Parras phone 422-3111 - see phone calls in Parras).

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  Figure 1: The completed system on the roof of Zaragoza #1 (2005).

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  Figure 2: Solar collector before glazing has been installed (2005).

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  Figure 3: Copper coils inside heat exchange unit (2005).

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  Figure 4: Alyssa and Mario insulating exchanger with wood chips (2005).

The complete solar hot water system involves heat exchange between three liquids:

• Solar collector fluid. The fluid which is heated in the solar collector (above, fig. 2) and which flows into the heat exchanger unit (see figs. 3 and 4), and back into the solar collector by thermosiphon. Antifreeze (anticongelante) is used as the solar collector fluid because it exchanges heat readily, and to prevent fluid from freezing in pipes and bursting them in winter. Alyssa Grassi and the 2006 project team have both reconsidered the necessity of antifreeze in this design, as Parras winters, while they very occasionally bring snow, are reportedly mild. Additionally, there are exposed pipes at Zaragoza #1 which carry water from the street, and these pipes appear not to have posed any problems.
• Intermediary heat exchange liquid. Water held in heat exchange unit. This intermediary fluid is necessary because of the hazards antifreeze can pose to human health. If, for some reason, the antifreeze circuit were to burst, its fluid would be released into the intermediary heat exchange fluid in the heat exchanger, not into the household flow.
• Household-bound flow.

City water flows into the rooftop system through exposed metal and PVC pipes.

2006 Repairs

When the 2006 project team first visited the house, the system was not functioning. The PVC pipe which should have been carrying antifreeze from the glazed (see definition below) solar collector to the heat exchanger unit had ruptured and therefore thermosiphoning could not occur. Despite the disabled status of the solar hot water system, the valves were still configured for water to flow from the street, to the roof, through the disconnected heat exchanger system, and into the gas water heater. The water’s flow through these pipes and systems actually heated it somewhat, presumably because of heat absorbed by the pipes and exchanger, all painted black.

Definitions

Glazed

Covered with glass. In the Zaragoza collector, this refers to the glass lid of the wooden box which holds the copper pipes.

Testing

July 7 2006, 10:30 AM, Cloudy 24°

July 8 2006, 10:50 AM, Partly cloudy 25°, Roof 28°

July 9 2006, 1:30 PM, Mostly sunny with scattered clouds, in sun: 36°, in shade: 32°

*measured after 20 liters of water had passed through system

July 8 2006, 4:30 PM, Partly cloudy 36°

*incorrect reading due to reversed polarity on thermocouple

Effectiveness of Home Solar System

The use of a solar hot water system is beneficial because it reduces the dependency on fossil fuels, and does not produce any pollutants. However, the system would only be considered a practical application in Parras if it was also cost effective. Many households in Parras use their hot water heater in a “on demand” fashion, the boiler is only turned on just before a shower. Natural gas is inexpensive, and the use is minimal, making it difficult for the solar system to compete with. The solar water system is capable of heating 28 liters to the desired temperature of approximately 40°C and then starts to drop off rapidly. The water from the street was 29°C so the system is capable of raising the temperature of 28 liters of water by 11°C. The cost effectiveness of the system was determined by estimating the cost of using the boiler to heat 28 liters of water 11°C. The density of water is 1 kg/l, so the mass of water heated is 28 kg, the energy needed was determined by:

Q = mswΔT

• Q = heat transfer
• m = mass of water
• sw = specific heat of water
• ΔT = change in temperature

Q = (28 kg)(4.186 kJ/kg*C)(11°C) => Q = 1289 kJ

According to gru.com natural gas water heaters have an efficiency of .54 so 2387 kJ are needed to heat the water. The amount of energy in natural gas is 50MJ/kg or 50,000 kJ/kg (from yesican-science.ca) the mass of gas needed is:

M = 2387 kJ ÷ 50,000 kJ/kg = .048 kg

The cost of gas in Parras is 25 kg for 280 pesos, so .048 kg costs .54 pesos. If the system was capable of going through a complete heating cycle two times a day is still would only save about a peso a day. At a cost of about 3,000 pesos it would take over 8 years for the system to pay for itself, which is most likely longer than the life of the system.

External Links

• Water Heater Efficiency
• Energy From Natural Gas