Solar-h2o

Current System

Our current hot water system uses LP gas and a hot water heater that was installed in 1998.

After working on an energy audit of Miller Farm (our house) it has become clear that we use a huge amount of energy to heat our water. Installing a solar hot water heating system then has become one of our goals, both because it does not require an actual conversion of solar energy to electricty- therefore eliminating the storage problem and the DC to AC conversion problem, and because the load is relatively consistant. Using solar power to heat water is a use of solar energy that makes a lot of sense for a house like ours as it is in a location that does experience a good number of gray days throughout the year and because heating is where we use the most energy.

Currently we have two retired disel fuel tanks in our basement. We are looking into how we prepare one or both of these tanks for use in our solar water system. We want to install a bladder in the tank in order to use it to store water, but protect it from the disel fuel residue.

Needed additions:formula for figuring out how long water needs to be in the pipes at different exterior temperatures in order to be shower temperature.

Experimental Solar Hot Water System

Basic Design

• Supplies Needed
  • (2) 1 1/4 inch female hot-water unit to hose connectors
  • a pump
  • hose?? (We found some hose in the green house that doesn't have end pieces)
  • sealable barrel (the barrels out back don't have lids that we can open. We need something sealed so that we don't turn the basement into a sauna)
  • copper coil
  • (2) hose to copper pipe connectors
  • tube-o-silicon
  • (long term) plexyglass or glass? and aluminum frame materials.

• Supplies Purchased
  • 12v Ag Sprayer Pump Product Information
  • 2 Red River 50ft. farm hoses guarentteed up to 180F
  • Teflon Pipe Tape
  • lots of pipe/hose connectors
  • flux
  • 25 ft 10-2# outdoor electrical wire
  • 30ft. soft copper tubing

• Supplies Purchaced-Addendum
  • nails and bolts for PV stand
  • self-liting butane torch
  • 12v connection cable for pump
  • metal juction box and wire nuts

• Supplies Acquired
  • scrap lumber
  • duct tape
  • 30? gallon black plastic barrell
  • various tools
  • 64 Watt solar panel
  • solar hot water heating unit

• Resources
  • Solar Panel Angle by time of year and latitude click

Heat Bank Storage and Heat Transfer Information

We plan on constructing a modified Thermal Store hot water system. We will use a closed loop solar heating panel and heat exchanger to heat water in a recycled 250 gallon feul tank, then cold main water will be passed through a second heat exchanger in this "thermal battery" before entering the farm's standard electric water heater. In this way the liquid in the solar heating loop, the liquid in the 250 gallon tank, and the potable water being heated all remain isolated from each other.

• Heat Exchanger Information
  • Ehren found a manufacturer that uses 24 sq ft of copper finned tubing ¾” to heat the cold main water. See the more detailed heat exchanger information at the bottom of the manufacturer's page.
  • References
    • http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12930
    • http://www.plumbingpages.com/featurepages/HWdpsThermstore.cfm
    • http://www.greenshop.co.uk/solar/documents/Conus-Cubus-info.pdf
    • http://www.heatweb.com/solar/solar.html
    • http://www.dhtnet.com/index.htm

Copper Pipe Heat Exchanger Test

In order to preheat the main water before it enters Miller Farm's electric hot water heater, we will route the supply through a copper pipe heat exchanger inside of a 250 gallon thermal bank, which is heated by our solor panel. This design was chosen because it is non-pressurized and simple to install; however, this design does create one significant challenge: we have a relatively short period of time to transfer heat to the main water traveling through the heat exchanger. Therefore, we must properly scale the heat exchanger (i.e. determine how many feet of copper pipe we need) if we hope to produce a significant change in the temperture of the main supply.

Data Collection

To collect heat transfer data for a heat exchange model we ran three tests using a 12" capped off piece of 3/4" copper pipe filled with cold tap water and a 5 gallon bucket filled with hot water. We held the pipe in the hot water with a pair of tongs and recorded the temperature of the water in the pipe every three seconds using a digital thermometer (carefully inserted into the pipe with plastic spacers so that it didn't touch the sides). Colin has created graphs of the change in temperature inside the copper pipe over time for the three test runs.

Flow Rate Through the Copper Heat Exchanger

Ehren found that the 12" piece of 3/4" diameter copper pipe holds approximately 112 ml of water. During our hot water audit of Miller Farm we estimated that the typical hot to cold water ratio for a shower at Miller Farm is 4:1 and the total flow rate is 2.25 gal/min. We can therefore estimate the hot water flow rate at Miller Farm to be approximately 1.8 gal/min = 6814 ml/min = 114 ml/s. This means that 112 ml will travel through a foot of copper pipe in 112 ml/(114 ml/s) = 0.98 seconds.

• • Now, Ehren played around with graphs of exponential functions and found that T=95-23*e^(-s/80), where T is temp. and s is time in sec., reflects (pretty well--we can certainly improve on this) the graph of the experimental data we collected in the first run of the experiment. Since the water for this experiment was about 97 degrees Fahrenheit and would most likely reflect the temperature of our storage tank in the mornings when folks are taking showers, lets see if we can scale an exchanger to match these findings. Our limiting factors are space in the tank, and price. The copper pipe costs a little less then $3.00 per foot. Since we'll need to buy some connectors let's just say an average of $3.00 per foot for the heat exchanger. Modifying our formula slightly we should find that Temp. Increase(TI) = 23-23*e^(-s/80). So, we can now find a formula that relates Temp. Increase to Length of Pipe, and Temp. Increase to Price: (Let L=length of pipe in feet and M=price in dollars).
    • L = (1/.98)*s = 1.02*s => s = L/1.02
    • and M = 3*L => L = M/3 => s = M/3.06
    • so, TI = 23-23*e^(-(L/1.02)/80) = 23-23*e^(-(M/3.06)/80)
    • Also, note that s = 80*ln(-23/(TI-23)), where 1/(TI-23) < 0. So given a desired temp. increase we can calculate how much it will cost and how much pipe we would need.
    • Thus, L=1.102*80*ln(-23/(TI-23)) and M=3.06*80*ln(-23/(TI-23)).
  • Now, if we take a look at dTI/dM = 0.094*(0.996)^M, we see that the slope of the change in Temp. over the change in Money is always less then 1. This means that every additional degree of increase comes at a higher price. So, we should base our dicision on available space and the long term payback of preheating the water before it goes into the electic hot water heater. For example, if we want to increase the temperature of the main water by 10 degrees Fahrenheit we need 1.02*80*ln(-23/(10-23)) = 46.56 ft. of pipe for $139.67. A 15 degree increase would require about 86.17 ft. at a cost of $258.52.