Monday, January 26, 2009

Sharing Hot Water- Does it work?

The Solar Harden Duplex is unique in many ways, one of them being a system set up to share hot water between the two buildings. Now that we have tenants in the Harden House, we are able to really analyze how this system is or is not working.
The SNAP House Solar Hot Water System: When the UT students designed the SNAP house one of the features they integrated was a solar thermal system. Solar thermal systems in basic terms takes the energy from the sun and uses that energy to heat water. This of course offsets cost associated with energy used to heat water which is attributed to 14 to 20 percent of the total electricity used in a home. Our hot water system has four main components: 16 Sunda's Seido evacuated tubes-solar collectors (image above), a 108.6 gallon hot water tank Stiebel Eltron SBB 400+ Twin Coil tank (image to the left), Seisco tankless water heater model RA-9 "flash heater" (image below), and a mixing valve.

The process for the system is as follows: solar collectors act as a low-resistance thermal conductor and heats a polypropylene glycol mix (similar to anti-freeze) that is slowly circulating through copper tubing in a closed loop system. This coil acts as a heat exchanger and is coiled back in forth inside the tank in order to heat the water. In case the water in the tank is not hot enough, an electric flash heater increases the temperature. If the water in the tank is too hot, the mixing valve works to maintain a decent temperature so that we do not burn ourselves (technical details are available from the websites listed below).


The hot water arrangement between the two homes
One of the experiments Blackland really wanted to try out is sharing hot water between the two homes by having only one large storage tank for both household demands. Therefore, the Harden house does not have a hot water tank but instead pulls all of its needs from our system. Hot water travels approximately 55 feet from the tank to the Harden shower head and 62 feet from the tank to the kitchen where a Harden resident is demanding hot water.

For billing and usage purposes, we record how much water they are drawing from our system everyday by reading a sub-meter on the unit (image above).

How is it working?
According to H2ouse.org (http://www.h2ouse.org/index.cfm), the national average for the daily per capita use for a shower is 11.6 gallons and for faucet usage (bath/kitchen) it is 10.9 gallons.

When assessing the recorded hot water usage at the Harden household (1 adult and two kids) their average usage per day has been 112 gallons.

Now, if we assumed that each family member took a shower and utilized hot water for each faucet for a day; then based on the measurements supplied by H2ouse.org, their daily usage would be around 60 gallons per day. However, what is being recorded is twice as much water.

Of course, there can be many reasons why their hot water demand is so large. One variable in particular is how far the water needs to travel to the point of use. When composing some intial test at the Harden House before the residents moved in; it would take approximately 3 minutes until temperature change occured through the Harden bathroom faucet. In addition, the Harden residents estimate that they wait a couple of minutes each time for the shower to get hot.

Thus, this arrangement is causing three problems that need immediate attention:

First, the distance that the hot water is traveling is wasting an enormous amount of water.

Second, during the winter time when the water is not being heated enough by the thermal tube collectors the flash heater is kicking in to heat the water. This is causing the SNAP house electric bills to be higher than ever before.

For example, for a day where the Harden household used 180 gallons of water and the SNAP household only 50 gallons (we purposefully did not take showers, do laundry, use the dishwasher, or use our mini-split HVAC system) our electric meter recorded 17 kWh. On a similar usage day before the Harden residents moved in our electrical usage recorded was 8 kWh. So, we can theoretically assume that the flash heater is driving our electric usage to be much higher.

Finally, in a series of days where it is overcast, there is limited hot water. Thus, water is not being heated by the solar thermal system and the tankless water heater is doing all the work. If the Harden house is using the hot water at the same time, there is only a limited amount of draw from the system and it (the flash heater) cannot handle the demand.

In other words, our shower faucet maximum flow rate is 2.5 gallons per minute. At 2.5 gallons per minute the tankless water heater can raise the water temperature 24 degrees Fahrenheit. However, if we are taking a shower and the Hardens residents are also taking a shower the ability of flash heater to increase the temperature of the water decreases almost by half.

Our suggestions:

We are meeting with Blackland on Thursday afternoon and we are going to recommend that they consider installing a separate hot water system at the Harden house. We will keep you all posted on the outcome.


Product Information
Solar Thermal Storage Tank
http://www.stiebel-eltron-usa.com/pdf/brochure_sbb.pdf
Sunda's Seido evacuated tubes-solar collectors
http://www.sundasolar.com/product_index.html
Seisco tankless water heater model RA-9
http://www.seisco.com/orderpage.html

Saturday, January 10, 2009

HVAC, Rainbarrels, Gardens, and Neighbors



It has been an active past couple of weeks here at the SNAP-Harden Duplex. This was the view from our home a couple of night ago!

First- the HVAC mini-split system is operable and working very well. The temperature takes awhile to stabilize itself in the space, so we need to anticipate when to turn on the heat in order to maintain warmth throughout the evening. We do this by simply paying attention to the weather and asking ourselves questions such as: is it going to be cold for an extended period of time, or is it going to be cloudy (remember our passive system works well too) for a long time? To learn more about our HVAC system, read the previous post.

The next project involving the HVAC system is to design and build screens that hide the air handler, but allows more airflow. Currently, there are wooden panels with 1/2 inch slates blocking the airflow. I will upload pictures of these soon.

Secondly, even with minor rainfall this past week, our rain barrels filled up and are running smoothly. There are two water tanks on the site. One is at the Harden House and the other is located at the SNAP house. Both are on the west facades of the building. They are Norwesco Freestanding Water Tanks (http://www.norwesco.com/page.cfm?menu=36) that have a capacity of 400 gallons each. This is plenty of water for the three garden beds that we are maintaining, and since we have been living here, I have only used the city water for 7 days.

Speaking of garden beds, I spent a good amount of time clearing and pruning the garden bed last weekend, and discovered some intruders- which could be either Cankerworms or a Cabbage loopers http://www.ces.ncsu.edu/depts/ent/notes/Vegetables/veg040e/veg040e.htm. My friend, Christi Greene, is a Master Gardener and she suggested spraying everything with BT (Bacillus thuringiensis). I took her advice and will probably need to spray again next week. They sure like our bok choy and spinach. However, even with those varmints in the garden everything is going well. The radishes are going strong and the broccoli is as well.

The final news we have is we now have neighbors at the Harden House. We haven't met them yet, but I have recorded their energy and water consumption data. This is very exciting data because now we can begin to see how these two homes are working together. For those of you who don't know, the SNAP house will be sharing it's electrical generation and solar thermal heated water with the 1930s house at the front of the lot.

Once the solar is generating power, we will really be able to see how a house like ours can help to keep utility bills affordable for two low-income families in the Blackland Neighborhood.