Saturday, October 1, 2016

First month of Solar Hot Water monitoring data

With the end of September we now have our first full month of energy monitoring data for the solar domestic hot water (SDHW) at 704B Wood Street (see for background).

Since September is an equinox month (average day/night) we can kind of assume it is representative of annual average sunlight.  So it is fortuitous that the first month of monitoring has been September as it gives us some indication of what annual performance might be. 

And the result?  Significantly less saving than predicted by the energy model.  The interesting thing is it does not seem to be due to system performance (which is performing better modeled), but due to a more insidious factor:  Conservation!  Basically the residents of the home are not using enough hot water to realize the potential of the system.  This is because there are a number of hot water saving measures designed into the house (including a very large drain water heat recovery system), that there are only 2 to 3 occupants in the house and lastly the residents are very energy conscience and therefore don't use a lot of hot water.

The first rule of energy conservation wins yet again:  reduce the consumption first before looking at efficiency, and only then (last) is renewable energy generation (which is what the SDHW system is.) 

Below is a graph of September's energy performance of the system. I'll explain the three bars and the preliminary findings below:

  1. The first bar on the left is the predicted hot water performance from the HOT2000 building energy model.  The model suggested that water heating in September would take 487 kWh of which 34% would be supplied by the solar system.  I don't know the details of what and how the model works, but I do know a few of the assumptions that help explain the difference.  HOT2000 standard reference defaults to assuming there are 4 people in the house (2 adults and 2 children).  It assumes they use 225 L of hot water a day at 55degrees C.  In the actual home there are 2 to 3 adults, they only use 125 L/day and the tank temperature is maximum 45degrees C.  So on water usage alone this house uses 44% less hot water than the model.
  2. The middle bar is the actual total energy usage of the hot water system.  In September a total of 277 kWh of energy was put into the hot water system, of which 41% was supplied by the solar system.  Therefore we see 43% less total energy input, which matches well with the reduced hot water usage recorded (versus the model).  But we did see in September 2016 that the solar contribution at 41% was substantively higher than that modelled.  
  3. The last bar (on the far right) is very interesting:  this is the amount of energy that actually supplied hot water to the tap.  The difference between this and the middle bar is energy lost as heat leaking off the storage tanks.  What we see here is only 131 kWh was used (of which 47% was solar supplied) to make hot water.  The remaining  146 kWh was presumably heat loss off the two big storage tanks.  Normally this would be considered "bad", but in the case of this super-insulated home, this might not be so bad.  This is because the house is electrically heated and with the high level of insulation, much of the "waste" heat off the tanks will be retained by the house.  This then displaces electricity that would be used to heat the house.  Of course, in the summer there really isn't any heating needed, plus the tanks are in the basement, a location that doesn't need to be as heated.  But none the less, the heat loss off the storage tanks isn't a total loss. 

Preliminary Findings

After one month of monitoring, it appears that the savings from the solar domestic hot water system are substantively lower than predicted.  This is not due to system performance, but due to low demand for hot water. 

Because this home is primarily in "first block" power (< 1,000 kWh/month), the electrical rates are quite low and therefore the savings (displaced electricity) are low.  At this time, first block power is approximately $0.109 / kWh.  At this rate, the solar system only saved about $12.49 in the month of September.   If that is representative of "average" savings, the total annual savings from the system are only about $150/yr.  That means it will be a very long pay-back for this system at these low electrical rates.

What I conclude from this is the solar hot water system seems to work well with good performance, however its value in a small, high-efficiency home, is low.  It is a technology that would be better suited to large homes with higher hot water demands or commercial/institutional buildings with large hot water demands. 

But, let's see how the rest of the year goes.

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