Solar Hot Water – In Practice – Will we change our ways?

In the previous post, we looked at the design and some performance elements associated with the solar hot water system and the gas/tankless assist parts as well.

In cloudy cold times it is clear that the system operates exclusively by depending on the gas assist from the tankless water heaters.  Cold water from the town of Tiburon enters the home through the solar hot water holding tank but we are then unable, on cloudy days, to use solar energy to raise the temperature of the water in this tank.  So then the tankless heaters pull from this storage tank water that is roughly the same temperature as supplied underground from the city.

On sunny days, the solar panels on the roof circulate a fluid that is both water and glycol (about 50/50) and deliver energy in the form of heat down to a heat exchanger and controller near the storage tank.  Water from the storage tank is circulated and raised in temperature in the heat exchanger from the water/glycol fluid that is hotter.  Over the course of 5-6 hours the storage tank can rise from 60F to nearly 120F on a very sunny day. Finally, when we pull water for showers on a sunny day, the warm water from the storage tank is then heated a bit more by the gas tankless heater.  Note that it is much easier for the gas to raise water temperature by 10-20F than it is by 60-70F.  All this is as it is supposed to be.

But the fastest rise in temperature occurs from when the tank is cold (60F) to approximately 100F.  Above 100F, the system will still increase the heat content of the storage tank but it happens more slowly.  Clearly inefficiencies set in above 100F.  Greatest of these is that the storage tank itself.  As water in the storage tank gets hotter it also loses heat all around the tank to the air. When the tank is above 115F it loses about 10F in 1-2 hours.  When the tank is 95F it loses 10F over probably 12 hours.

So it is best for us to take showers and use hot water when the tank is hot and when there is still more heat to be supplied from the sun so that we can store it over night for any early morning showers as well.

And the question asked by this essay is how much are we able or willing to change habits to accomodate the free energy from the sun?  Many people are accustomed to having a shower to wake themselves up and as the first preparation for their work day.  Clearly this is not the best time for that shower!

In many homes showers are also taken in the evening after a workout, after coming home from school (sports practice, for example) or just to relax before bed and these showers can have most of their required energy supplied by the solar hot water system.  But then the storage tank will be fully depleted of energy and of little use for any morning showers since no sun power will be available until the sun is well risen in the morning.

How many of these catch 22’s will we find as we move to lifestyles where more of our energy is supplied by renewable energy sources?  How much change in our habits will we make to milk the natural efficiencies in these systems to their fullest?

Tell me what you think.

Posted in Essays, Green Perspectives, Personal Stories

Green Home Details – Hot Water

In a previous post were summarized the various green elements of our home in Tiburon. Now let’s dive into the details. This post will deal with how we make hot water including system design, estimated savings and early system performance data.

As mentioned earlier. the overall house is built to have 2 separate living quarters which we call the home and the apartment. When we first purchased it, the home had 2 people living in it and the apartment had one person living in it. Both the home and the apartment consumed natural gas for home heating and for making hot water for showers and sinks. Overall, with careful use these places together used approximately 120 therms of natural gas on average per month – more in the winter and less in the summer when only the hot water was called upon.

The gas furnaces were typical forced hot air systems and the hot water was made in 2 separate tanks – one of 50 gallons and one of 40 gallons.

One way to look at this is that the house was using about 40 therms per month per person. Today we have 4 people living in the house – 1 in the apartment and 3 in the home.

One goal we had for the overall house was to cut our carbon emissions as much as practical. So we believed it would be possible for us to make substantial amounts of electricity from photovoltaic panels and so we converted the gas home heating system to heat pumps. This eliminated about 50% of the overall gas load but shifted it to the electrical side of the equation.

Next we decided that if we were to use solar energy to make much of the hot water we required, that we would only install one solar assisted hot water system to serve both the home and the apartment. After further investigation, we learned that tankless hot water heaters have numerous advantages over the older style tank based systems.

Some of the benefits of a tankless system are –

  • no use of gas during periods when there is no demand. For example, if you go on vacation, a tankless system will use no natural gas, but a tank system (unless you turn it off) will keep a tank full of hot water ready for use even when you are not there.
  • tankless systems work well with solar assist because they make water even easier if it is preheated like naturally happens when the sun shines.
  • tankless systems can be designed to use multiple heating units in series and this has the effect of increasing the capacity of the system; however, if such a system has a light load the extra unit will remain off and use little or no energy. We opted for this approach as there are times when the house has as little as 2 people but there are also times when there are 8 or more people staying there. So the load on the hot water system is highly variable but we never run out even with everyone taking showers at the same time. Cool. But still highly efficient.

Domestic hot water typically accounts for 15-20% of your home utility bills (1), (2), (3).

We also have a gas fired cook top or stove which has 5 burners, 3 normal and 2 higher heat for Woks or large cooking. We estimate that our gas usage splits about 75% for hot water and 25% for cooking. (Most of the energy we need for cooling comes from electricity through the microwave oven, toaster, steamer, coffee machine, etc.)

So if our average monthly usage used to be 120 therms when we had the gas furnaces and the tank style hot water heaters and no solar help. Our goal was to cut our natural gas usage by 90%.

Here is the system block diagram or schematic for our hot water system for the home.

Block Diagram

Block Diagram

The way this system operates, we have 2 hot water panels on the roof and they have plumbing that puts them in series one after the other.  Over a typical sunny day, the heat energy derived from the panels is used to warm water in a 120 gallon storage tank.  As we use hot water, it is pulled from the storage tank and passes through a pair (also in series) of tankless hot water heaters that are natural gas fired.  If the water in the tank is not warm enough for our general use (showers and such) then the tankless heaters warm it further before it is distributed by the home plumbing.  Typically the water is warm enough so that the sun is doing most of the work.  But during a prolonged cloudy period the tankless water heaters can do all of the work by themselves.

With the system described, we make about 75% of our hot water from the sun and the additional 25% of the hot water from natural gas.  Because we also took out our gas furnaces in the home and replaced them with heat pumps (air heat exchangers and not ground source) our overall natural gas usage had decreased from an average of 100 therms per month (annual basis/12) to around 10 therms per month.

So now let’s take a look at how this appears.  In the early morning following a day where the sun was shining but where some showers were taken the previous night and where we did some laundry or dish washing, the system looks like the following –

Hot water system before sunrise

Hot water system before sunrise

You may notice that the collector output temperature is actually warmer than the storage tank, this snapshot of the system was taken just a few minutes after sunrise.  The panels are warming up but they do not have sufficient heat to start up the pumps and begin the heat transfer.  The next snap shot, taken 10 minutes later crosses this threshold –

Hot water system as sun starts heating

Hot water system as sun starts heating

In our system the pumps are turned on and heat begins to transfer when the panels are 18 degrees F warmer than the tank.  And this transfer stops when the panels are 2 degrees warmer than the tank.  Because of the way the tank begins heating you see an immediate small temperature rise in the water temperature.  Next let’s look 10 minutes later –

Hot water system 10 minutes after start up

Hot water system 10 minutes after start up

Again, you can see that the panels are now steadily supplying energy in the form of heat and that the temperature of the water in the 120 gallon tank has risen by about 2.5 degrees over that time period.  After 1 hour, the system now reports –

Hot water system 60 minutes after sun warming begins

Hot water system 60 minutes after sun warming begins

Now the tank has risen about 8 degrees in a period of 1 hour.  therefore we might expect a temperature rise of about 24 degrees in 3 hours –

Hot water system 3 hours after sun warming begins

Hot water system 3 hours after sun warming begins

We get actually 23 degrees which shows that the system behaves relatively linearly while the tank temperature is not too hot.  After 4 hours the tank temperature rose another 8 degrees to around 102 F.

On a fall day – this data was taken on October 11, 2008 – the sun rose about 7:30 am and reached a peak input to the system around noon.  By 3 pm in the afternoon, the angle of these hot water panels begins to be inefficient and little heat gain occurs after that time.

Another point that will come up again and again in this series of essays is that I have found this ability to visually see teh performance and operation of the system most helpful.  Early on, the system had some plumbing problems and lost the working fluid that brings heat down from the panels to the contol system and eventually into the tank.  These might not have been spotted without monitoring. Later we had problems with the control electronics and a circuit board had to be replaced.  Again, without monitoring, I do not believe we could have seen the problem and fixed it quickly.

The system elements in our hot water components are a standard, well insulated 120 gallon tank, the hot water panels, control and monitoring electronics and heat exchanger are all made by Heliodyne, and the tankless water heaters were made by Takagi and all of the internal hot water pipes had pipe insulation to retain the heat as it moves around.

To close this essay, here is the overall performance of the system (but including gas we use for cooking as well) –

Gas usage from August to mid-October

Gas usage from August to mid-October

As you can clearly see our natural gas usage has gone visibly down since the system was installed and we began monitoring in early August, 2008.  More interesting is the fact that the peaks that are visible in early September and early October were times with the solar hot water system was broken.  Now it has been running stable for a couple of weeks and we believe it will be reliable in the future.

This is one of the graphs you will soon be able to see in real time on the SWG website under the Tiburon Home Tab.

Stay tuned!

Posted in Essays, Green Perspectives, Personal Stories

Design of a Green Home

In the last blog post, I wrote about teh home where I live in Tiburon, CA.

Before embarking on the ambitious remodeling project that lasted 18 months, we decided to live in the existing home for about 1 year so that we could experience all 4 seasons in it as it existed.  The home consisted of essentially 4 areas –

  1. the original home, built in 1971, which had a couple of bedrooms, 1.5 bathrooms, and separate kitchen, living room and dining room.
  2. an apartment that could be rented out that had 2 bedrooms, 2 bathrooms, a galley kitchen and combination living room and dining room.
  3. an addition to the home that added 2 more bedrooms and a more formal entry and staircase
  4. an outside that included some landscaping and an irrigation system to water it all using water from the Marin Water District.

Because of when the original home and apartment were built – clearly in a time of cheap energy – these spaces were poorly insulated, had single pane glass and doors and windows that made the home drafty on windy days.  The addition to the home was better built but still less than we thought acceptable today.

Our goal in the remodel was to first save substantial energy in the design through better insulation, better windows and doors, better management of lighting and more efficient appliances.  This included insulation in the ceilings of the home where typically sometimes none existed before, insulation also on inside walls for sound proofing but also so that some sleeping rooms could be cooler than the core of the home.

We also worked with the builder to insulate the underside of the home.  The home is situated on a very steep hillside, and sits on steel stilts that leave its underside substantially exposed and this area receives string wind and storm buffeting in the winter/raining season.

By better sealing each of these areas, the house was made tighter and warmer through out the year.  You may ask what happens when the summer days are very hot?  In Tiburon, because we are on the water, this happens rarely.  But when it does, we are able to open the house and then the normal winds help us stay cool and comfortable.  (The remodel saw us change from gas furnaces to heat pumps.  This gave us air conditioning ability but we have never used it to keep cool.)

We also designed in more glass facing east and south so that the home would have substantial solar passive heat gain.  The home has roof over hangs that serve to keep the sunlight on the decks during the summer months but in the winter, as the sun’s rays have a lower angle to the home, the sun comes naturally into the home.  This really helps the heating in the winter.

Of course there are solar photovoltaic and hot water panels on the roof to make energy for the home.  And we also have cisterns below the house.

In future essays, I will present details about the home.  In this way, we can share with you our decisions and how they have worked out.  Much of what we did was relatively inexpensive and has rapid pay back.  Here is the essay roadmap over the next few weeks –

  • Solar hot water
  • HVAC and insulation
  • solar photovoltaic – electrical power
  • lighting
  • appliances
  • Audio/Video
  • green materials
  • monitoring and control systems
  • luxuries – pool, hot tub, fire place

Hope you enjoy these pieces.

As I am able to complete the series, the home should be a part of this web site.  Real time data for our daily electrical, water and gas consumption as well as data for how much energy we generate and how much water we store.  The goal is to make it a real time score card for how the home is performing.

Stay tuned!

Posted in Essays, Green Perspectives, Personal Stories

Chasing Phantoms

There will be a new part of the Small World Group website soon. It will cover the operating model and actual performance of my new home in Tiburon, California, USA.   Tiburon is one of the sunniest places in the USA and it has a temperate climate where it is rarely above 80F (27C) or below 45F (7C). The home has most of the technology to be “green”.

But key to me is can we go so low in carbon use to say that my family has a carbon neutral footprint? One of the key insights that I had read but never fully understood was how important it can be to track all of the electrical uses in the home. Today there are many “loads” that have some residual current flowing even when the device or service seems to be completely turned off. Some examples of these phantom loads are computers running screen savers, outside lighting that is on all night; more subtle ones include cable or satellite TV boxes that are on continually to provide instant service. Why are these so important? Let’s look at a few cases to help you understand better –

  • Consider outside lighting that you may be using to feel safe or that is helpful to guests and visitors who arrive after dark. Suppose you have 8 lights each using only 60 watts each. That is a total of nearly 500 watts. if you use a sensor to have these turn on at night (i.e., the sensor detects when it is dark out) then you will typically use this 10 hours per day (average of summer and winter) and so you use about 5 KW-hrs per day or 150 KW-hrs per month. This would be 50% of what PG&E, our local electric power supplier calls a base load. All for lighting that is rarely of benefit. By exchanging these lights for compact fluorescents the power per bulb for the same light could drop by 75% so the monthly load would be perhaps 40 KW-hrs. But by putting the lights on a combination of light sensor (to know it is dark outside) and a motion detector (motion detectors enable the lights to turn on only when there is someone outside) as well you could cut this likely 99% so you would only use perhaps 1 KW-hr.
  • Next let’s look at the CATV or satellite box that connects your television or display to the service provide’s entertainment. Today it typically has multiple memory devices. Some information is downloaded from the system into active computer memory to help it know channels and services available now and for the next 2 weeks. The “box” uses anywhere from 30-200 Watts continuously so again this device would account for between 1-5 KW-hrs of power per day or 30-60 KW-hrs per month. This one box can cost you 20% of the normal or low cost power from a provider like PG&E. A better answer is to decide when you actually watch TV and to either turn the entire system off manually or purchase a small timer outlet from your local hardware store. By using this device you could turn all of the home entertainment gear off from mid-night to perhaps 4 pm the next afternoon. This again would save 67% of the load and likely provide you with the same perceived level of service. Of course you would not tape an episode of “Law and Order” at 4 am in the morning.
  • Another type of phantom load are personal computers and the equipment associated with them.  If your internet service provider is either cable or the telephone company (DSL) you have a modem to send and receive data from the transmission wires or cable, you have a router to make the service available to multiple people or users in the home, then you have printers, computers and other devices that use this service.  If your leave your computer on when you are not using it (there have been recommendations that this is “good” for the computer …) and even if you are using a “screen saver”, the computer gear probably has not been put in a low power state.  So all of the above gear probably uses about 200 watts of power steadily.  Again, this would be about 5 KW-hrs per day or about 150 KW-hrs per months.  The simplest way to save here is to use the computer’s sleep mode.  This will drop the computer’s power usage down by more than 95% but it will likely also turn off the display screen to a low power state as well.  For the system I am using to write this my savings are about 140 watts.  To go further, you would need to agree in your home that you are unlikely to need internet access certain hours of the day say from midnight to 6 am and from 9 am to 4 pm.  Then you could put a timing switch (cost about $10 at any hardware store) to control the power to the power strip for those elements and save another 50-100 watts depending on the age and extent of your equipment.

As I have chased the phantom loads inside the remodeled home, our energy performance has substantially improved.  But it requires discipline and some careful thinking through things.   It is possible to make real mischief among teenagers, for example, who might be using the internet when you think they are not!

More on this later … and some real data as well.

Posted in Essays, Green Perspectives, Personal Stories