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Solar Hot Water Heating

In April 2007 I had Solar Hot Water heating installed in my house in Wellington.  On this page I am documenting my experiences and the performance I have received over a year of detailed monitoring.  In summary:

Photos of my installation

Solar panel on roof Closeup of panel

Display                                        Cylinder

As the photos show, I have a 30 evacuated tube solar collector mounted on the roof.  It is facing north west so is not quite in the optimum position, but the loss of efficiency is compensated by additional tubes (ie 30 instead of 20).  I have a display unit mounted on the wall in the kitchen for easy monitoring, and I have a 270 litre solar-ready cylinder installed in the basement as a holding tank ahead of my existing 180 litre hot water cylinder which is over 40 years old.  When it finally dies I will remove it and just use the 270 litre cylinder. 

The installation was done by Solar Advantage (no longer in business) , and there is more information on their website on the hot water cylinder configurations, and the evacuated tubes.

Installation diagram

The diagram below shows the main components of the system, most of which are visible in the photos above. I have frequently been quizzed on the benefit of the 270 litre solar holding cylinder ahead of the 180 litre main cylinder - there are pros and cons to this, which I will discuss in more detail later.

Solar installation

Solar insolation and collector angle

It won't work without the sun!  NASA provides solar insolation tables, and for my location in Wellington, New Zealand, the details are given below.  The measurement unit is kWhr/m2 per day.  The evacuated tube dimensions are 1800 x 58 mm x 30 = 3.132 m2 of collecting area.

Country City Latitude Longitude Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Avg
NZ Wellington 41' 17" S 174' 47" E 6.27 5.31 4.17 3 1.95 1.54 1.74 2.46 3.66 4.7 5.73 6.01 3.88

However, the panel is facing WNW rather than due north at the ideal angle of 45°, and this reduces the received radiation by about 20% (see table below).  The collector tubes are not 100% efficient, but efficiencies are quoted at around 70%.  Taking all this into account, this means that I can expect to receive approx. 2.7 kWhr of energy during the middle of winter, up to 11 kWhr during midsummer, and on average about 6.7 kWhr per day throughout the year.  My hot water requirement is 3.5 - 4 kWhr a day, so the system should be capable of supplying my needs during much of the year, providing the sun is shining.

I obtained the following table from the Sunshine Solar site.   My panel is not positioned optimally, but I don't expect the loss of efficiency to be more than 10% - 20%.  My collector faces north-west (probably about 300° and the slope angle is probably around 25° - 30°.

Efficiency loss through non optimal collector position
Collector facing Slope from vertical
20°
40°
60°
80°
90°
West
270°
15%
20%
28%
40%
47%
 
300°
8%
8%
14%
27%
35%
 
330°
2%
1%
4%
20%
29%
North
0°
3%
0%
6%
20%
30%
 
30°
6%
5%
12%
26%
35%
 
60°
12%
14%
23%
35%
43%
East
90°
20%
27%
36%
48%
54%

 

Actual performance

Please also refer to the graphs on the adjacent tab.   I spent the first year from May 2007 to June 2008 monitoring the performance of the installation.  This included using a datalogger to record exactly how much the 1.5 kW heating element on my hot water cylinder was really used.  No claims or guess work or estimate - I have solid evidence of actual performance.  Of course, my hot water usage does vary on a daily basis, and the weather has an impact on performance, but average over the weeks and months I have built up a good picture as it applies to myself and my installation. Performance for other households in other locations could well be different. The parameters of my installation are:

The solar collector needs to supply at least 12.4 MJ of energy, but the situation is a little more complex because of the holding cylinder I have ahead of the main cylinder.  Solar energy is transferred to the holding cylinder (which also has losses of about 4.5 MJ a day), and the energy in the water is only transferred to the main cylinder when I actually use hot water.  The table below shows the effect of this energy transfer.

Incoming temp Celcius 13 25 35 45 50 55 65 75 83
Operating temp Celcius 55 55 55 55 55 55 55 55 55
Celcius rise Celcius 42 30 20 10 5 0 -10 -20 -28
Water volume litres 43 43 43 43 43 43 43 43 43
Energy needed kJoules 7,549 5,392 3,595 1,797 899 0 -1,797 -3,595 -5,033
Daily losses kJoules 5,000 5,000 5,000 5,000 5,000 5,000 5,000 5,000 5,000
Heater element kW 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
On time Minutes 139 115 95 76 66 56 36 16 0
Savings % 0% 17% 32% 46% 53% 60% 74% 89% 100%
Units / day   3.5 2.9 2.4 1.9 1.6 1.4 0.9 0.4 0.0
Cost / day @ 19 c/unit   0.66 0.55 0.45 0.36 0.31 0.26 0.17 0.07 0.00
Cost per year $ $241.74 $200.19 $165.57 $130.94 $113.63 $96.32 $61.69 $27.07 $0.00
Savings per year $ $0.00 $41.55 $76.17 $110.80 $128.11 $145.42 $180.05 $214.67 $242.37

Explanation: the holding cylinder is heated by the sunlight and stores energy.  Assume that on a typical day it heats the 270 litres of water in the tank from 25 to 45 Celcius (imparting 4.18 x 20 x 270 = 22.5 MJoules of energy into the cylinder).  At the end of the day I use 43 litres of water, but instead of the water being drawn into the main tank at 15 Celcius, the water is provided at 45 Celcius.  This means that the heating element needs to heat the water by 10 degrees instead of 40 degrees, hence less power usage.  The heating element thus needs to provide 4.18 x 10 x 43 = 1,797 kJ plus 5,000 kJ for the losses of the cylinder.  This is a 46% saving compared to no solar heating.  However, for zero power usage the inlet water temperature needs to be raised to 83 Celcius.  If I increase hot water usage, the maximum inlet temperature for zero power usage drops because more hot water is being added to the tank to compensate for losses. 

Cost Savings

The detail is below, but at a high level, instead of paying $250 a year for hot water heating I paid about $123, so a saving of about $127 or just over 50%.

Cost of installation - $7,200 plus a further $750 for City Council Building Consent.  So payback period > 50 years at current power prices!

The table below shows my actual power usage over the past 3 years.

Period
Units
Power charges
June 2005 - May 2006
4,020
$834
June 2006 - May 2007
3,880
$921
June 2007 - May 2008
3,185
$849

The table below shows how power charges have varied over the past 3 years (generally cost increases are made in December each year):

Year
Line charge per day
Charge per kWhr
2005
33.33 c
16.05 c
2006
33.33 c
17.33 c
2007
33.33 c
19.03 c
2008
33.33 c
19.94 c

There has been a significant increase over the past few years, but I have seen a $73 per year decrease in charges over the past year despite a 4.5% increase in power costs per unit.  Of course, I have other appliances and there are other factors which influence power usage (like a cold winter).  Without solar heating, hot water would account for approx. 1/3rd of my power charge.  The fixed line cost of $121 a year is also signficant. 

For me, saving 50% - 60% of $250 isn't a huge saving, but a larger household would expect to save a similar percentage so if they were spending $500 a year on hot water they could expect to save at least $250 a year.  However, you would need to be spending $1500 - $2000 on hot water annually in order to get a payback within 10 years on the initial capital investment!

General Comments

Am I happy with the installation?  Yes.  It isn't all about financial returns. There are many other things you can spend your money on - cars, boats, barbecues, kids - all consume money without any payback at all!  If money is your prime motive it is also worth investigating other methods of heating water including gas, wet-backs, and heat-on-demand (by gas or electricity). 

A 40 Watt water pump is required to circulate water from the collector to the holding tank.  This could easily be powered by a small solar electric cell installation and there provide me with hot water in times of extended power outage.

Some notes regarding my installation:

Government Grant

What Government Grant?   Yes I know there is a web site that talks about it, but don't expect to actually get it.  As far as I know, nobody has actually received a $500 grant, and the eligibility criteria are so tight that few people will ever actually receive it - see recent news item below.  It is a pity the government isn't doing more to encourage the installation of alternative energy.  These headlines appeared in the NZ Herald, Friday 15 February 2008:

"Solar hot water subsidy off to slow start
NZPA, Friday February 15, 2008
Green Party co-leader Jeanette Fitzsimons is confident more people will take up a $500 solar hot water subsidy, despite no one taking it up in the past year."

I believe that there is a new $1000 grant which may be easier to obtain, but check eligibility carefully first to avoid later disappointment.

FAQ and Q&A

  1. What happens if the water boils?
    I switched the pump off mid-winter to see what would happen. The water did boil, even in mid-winter. If water is not continually circulated through it can soon reach 100 degrees. The water boils off through a vent on the collector. No damage will result to the tubes or collector. The worst that could happen is that an air pocket could develop in the pipe and stop the small 40 Watt pump from circulating water, but this didn't seem to be a problem for me.
  2. What happens during a frost?
    The controller circulates water from the tank back through the collector if the temperature at the collector falls below 5 Celcius.  Again, even in a frost no damage would happen to the tubes, the worst would be a burst pipe to/from the collector due to the water freezing.
  3. Does the collector need regular cleaning?
    The round surface of the evacuated tubes are cleaned by rain. I haven't needed to clean them yet.
  4. How much water circulates through the collector?
    Approximately 8 litres of water is heated at a time.
  5. How frequently does the pump operate?
    In mid-summer full sunshine with the controller set to switch the pump on at 10° Celcius and off when 5° Celcius above the temperature of the main tank, I observe the pump to switch on for 2 minutes, and then off for 2 minutes.  At other times the off interval is greater than this.
  6. Is the pump noisy?
    It is very quiet and can hardly be heard in operation when standing right beside the unit.
  7. Is the holding cylinder ahead of the main cylinder advantageous?
    The jury is still out on this one. I feel it is advantageous in winter in Wellington, but a hindrance in summer (ie more energy would be transferred to the main tank in summer, especially if hot water usage was low and thus not drawing a lot of water from the hot holding tank into the main tank).
  8. What is the highest temperature of the hot water cylinder?
    The 270 litre hot water cylinder was raised to 84° Celcius in late November when I was away for a couple of days and not drawing any water off.  It reached the low 80's also in January and mid-February.  This means that the collector temperature was reaching mid 90's.  In mid-winter the water will still boil if it is not circulated back into the tank with cooler water replacing that at the collector.
  9. Are evacuated tubes better than flat panel solar collectors?
    I have seen performance graphs suggesting that performance is similar in hot climates. I think that evacuated tubes are better in Wellington (and further south). There have also been well publicised quality issues with some flat panel collectors, and these problems would not occur with evacuated tubes. However, the evacuated tube rack is uglier than a flat panel collector, but it is only the neighbours that get to see it!

 

© In the Light, 13 May, 2011 , Disclaimer, Son of Suckerfish drop-downs from HTML dog