[Greenbuilding] Opinions on electric tankless HW heaters?

Norman Feldman nfeldman at fountainhouse.org
Sat Apr 23 15:12:24 CDT 2011


Bob, 
If and when you add a solar water heater:

NTS-Solar has directflow evacuated tube collectors that output heat transfer fluids at over 400F -- fewer collectors needed for DHW and space heating. I don't have personal experience with these collectors but have talked with system designers and know of projects where they're in use.
http://www.nts-solar.com/

If a solar thermal system uses glycol as the heat transfer fluid, the glycol will stagnate if kept at over 400F for awhile (I think awhile means a few hours). One workaround is to use heatpipe collectors instead of directflows. NTS heatpipes limit output to 320F, below the temperature of stagnation. Narva manufactures the ev tubes NTS-Solar assembles and distributes. Their website has brochures that describe directflow and heatpipes.
http://www.narvasolar.com/en/home.html

Another way to avoid stagnation would be to use thermal oil instead of glycol. It's possible some building codes don't allow thermal oil.

A third way to keep a system from overheating is to have sufficient demand for collector heat during the summer. This might also be the most cost effective system. Typically solar thermal systems get sized for the summer load. If you size for the winter heat load -- enough collectors to provide DHW and space heating -- you have too many collectors if DHW is the only thing you need heat for in the summer. Solar air conditioning can use heat in a hot water-driven absorption chiller. Here are some references to up and running projects using Yazaki, Broad and Thermax hot water chillers.  
On the NTS-Solar homepage, scroll down a screen to where it says, "Largest Solar Heating and Cooling installation in all of Canada is now OPEN!!"

This company has done a few solar air conditioning projects with Yazakis, some, I think, residential. I don't know what collector they use.
http://www.solarpanelsplus.com/solar-air-conditioning/

80 ton system at Steinway Piano factory in Long Island City, NY using Broad chiller/heaters.
http://www.treehugger.com/files/2008/12/steinway-solar-air-conditioning.php

Climatewell has a SolarChiller with a higher COP than Yazaki/Broad/Thermax (3 in contrast to their 1) which means fewer collectors needed for the cooling load. Also, it stores heat overnight in molten salt. Climatewell developed their chiller about five years ago, has won a number of awards (2007 best invention of the year, GE Ecomagineering) but hasn't been distributed in North America. As of last fall they were looking for test sites here. 
http://www.climatewell.com/index.html

You could also use collector heat during the summer through a CoolEnergyInc. SolarHeart Stirling engine, which is used as part of a system that produces heat and/or electricity -- in summer excess collector heat is used by the Stirling engine to produce electricity.
http://www.coolenergyinc.com/

This email is already too long. If you want, I will send further references off-list.

Norman


-----Original Message----- 
From: Paul Eldridge
Sent: Friday, April 22, 2011 8:27 AM
To: greenbuilding at lists.bioenergylists.org
Subject: Re: [Greenbuilding] Opinions on electric tankless HW heaters?

Hi Bob,

You mentioned that your electricity consumption increased by more than a 
thousand kWh per month when you operated your electric water heater 
continuously, which suggests a hot water demand in excess of 33.0 kWh per 
day.  The Energy Factor rating for a newer, conventional model would be in 
the range of 0.92 to 0.94, which means the standby losses are roughly equal 
to that of a 40-watt light bulb, or less than 30.0 kWh/month.  [Note that if 
you were to add an insulating blanket, you could probably cut that by a 
third or more, but first check with the manufacturer to ensure this won't 
void your warranty.]  After subtracting these standby losses, some 970+ kWh 
would be attributable to water heating; assuming an inlet temperature of 10C 
and a supply temperature of 50C, that's roughly 180 gallons or three and a 
half tank's worth of hot water demand per day.  Are you sure you were using 
this amount of hot water?
+--------------------------------------------------------------------+
We use to heat our DHW with oil (there's no natural gas service in our area) 
and have since switched to electric.  We installed a small, 70-litre, 
115-volt, 1.38 kW model that is plugged into a Kill-a-watt power monitor. 
Our two person household averages a little less than 5.0 kWh per day, 
including standby losses, and we both shower daily, wash clothes in hot 
water and operate the dishwasher twice a week.  We've made a concious 
decision to wean ourselves off fossil fuels to the greatest extent possible, 
and as part of this purchase 12,000 kWh/year of 100 per cent renewable 
energy through Bullfrog Power.  That $20.00 a month effectively offsets 115 
per cent of our home's total energy needs, i.e., space heating (ductless 
heat pumps), DHW, cooking (induction), lighting, appliances and plug loads. 
>From our perspective, electricity is our best option.

Cheers,
Paul

----- Quote -----
  We put in an off the shelf mid-price electric water heater, and added an 
on off switch, so most of the time it is off and we turn it on maybe twice a 
week.  People periodically tell me that that doesn't save any energy, for 
various reasons, and while it is true that I am not a scientist. . . as an 
experiment we left it on 24/7 a couple of months in the fall when it wasn't 
especially hot or cold and our bill went up over a hundred dollars each 
month (more than a thousand kilowatt hours) so I would say that the simple 
on off switch does in fact save energy. Not to mention giving my poor frugal 
heart pains when I opened the bill. The plan when we get more money is to 
add a solar water heater.

   Bob Waldrop, Oklahoma City




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