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Envr 225 - Sizing a Solar Hot Water System for Hot Tub

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ENVR 225

Fall 2010

Gavin Peterson

Final Project Assignment

1)The Drawing:

The drawings are attached to this file. The schematic depicts the fluid flow and components that are incorporated into the system. The house view depicts the physical locations of the main components and where the panels will be mounted.

2) The Narrative:

The load that my design is attempting to serve is a single 225 gallon indoor hot tub. The hot tub will ideally be kept at 107 degrees F, while the average inlet temperature is 75 degrees F. The system I chose is an active direct system, which is a commonly used type for pool heating. This system will pump pool water through the filter up to the solar panels, where is will be heated directly and pumped back down into the pool. I chose to heat the pool water directly instead of using some heat transfer agent for several reasons. Heat transfer agents can irritate the skin and be harmful if swallowed; it would be bad if it contaminated the pool. Direct heat is in my opinion the most simple and safe way to heat a pool. The system can also be heated by gas through the pre existing water heater, which is now its back-up energy source. Sensors in the system dictate whether to run the water through the solar panels, or directly through the water heater. When the pool temperature exceeds the temperature of the collector, the 3way valve opens, forcing the flow to bypass the collector loop. I used this type of control device because, when operating properly, the controller automatically adjusts to compensate for changing environment, monitoring variations and other factors. The flow will pass first through a stainless steel low head pump. The water will be filtered and pass through a spring check valve, so there is no backwash. It will then pass through the auto 3-way valve and ideally be directed to the collector loop. As the water moves to the solar collector it passes a ball valve (for manually closing input) and a drain valve(for manually draining the system). The water will reach the panel and the travel to the top as it increases in temperature. A vacuum breaker, pressure relief valve and air vent should be installed at the highest point in the system. The vacuum breaker prevents thermosiphoning; there is a plastic flap that is held closed by water pressure, when the pressure drops it allows air in as to prevent backflow of water. The pressure relief valve ensures that the piping does not explode in the collector loop (even if the loop is closed and the fluid is overheating). The air vent allows excess air to escape (always placed on the highest point of the system) without allowing air into the system. The heated water will then pass the freeze protection valve, and the collector sensor. Where it will be pass the 2nd pump through another spring check valve and ball valve to the hot water tank.

3) Load Calculations:

Desired Temp = 100*F

Source Temp= 75*f (average Santa Fe inlet temp)

107*F - 75*F = 32*F

32 x 8.3 = 265.6 per gallon

265.6btu x 225gallons = 59,760 BTUs\day

Load = 59,760 BTUs\ day

Climate - Clear day - 1000

A Glazed Flat-Plate panel will generate an average of BTUs 44,000 BTU\day

44,000 x 2 = 88,000

Efficiency = .7 x 88,000 = 61,600

This system will require 2 EC-40 glazed flat-plate collectors to generate 61,600 BTUs. That will be enough BTUs to raise 225 gallons of water (size of hot tub) from 75*F (inlet temp) to 107*F (desired temp)

4) SRCC and Climate Data:

The SRCC and climate data were used from the hand out. Attached will



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