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Thermal Accumulatorsby John Siegenthaler, P.E.

June 1, 2008

Another European concept that will eventually show up in North America.

A significant advantage of hydronic heating is the ability to use heat from a ide variety of heatsources. !hese include traditional devices such as gas" and oil"fired boilers, as ell as alternative

sources such as geothermal heat #um#s, solar collectors, ood"fired boilers and aste heat recovery.

!he availability of heat from some of these sources is s#oradic. $bviously, solar in#ut only occurs

hen the sun is out, and heat from a ood"fired or #ellet"fired device is only available hen the fire is

burning. %eat in#ut from a geothermal heat #um# might only be economically desirable during #eriodsof lo off"#ea& utility rates. !o further com#licate matters, the ater tem#erature su##lied from these

heat sources can vary idely.

So ho does one set u# a system to accommodate to or three of these heat sources' $ne a##roach

ould be to install an elaborate collection of controls. !ogether, these devices could decide hich heat

source has #riority de#ending on the current and antici#ated loads, the cost of o#erating the device, the

tem#erature and heat out#ut rate of each device. !he logic ould also have to address the avoidance ofshort"cycling the various heat sources.

$nce the logic is determined, attention ould turn to #i#ing all these devices together. !his #i#ingould #robably include devices such as mi(ing valves, #rimary)secondary #i#ing, heat e(changers and

multi#le storage tan&s * the or&s. !his a##roach gets com#licated and e(#ensive at the same time.

Figure 1

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+uring my visit to the ostra -onvegno E(#ocomfort in ilan, taly, this #ast arch, it as evident

that several Euro#ean manufacturers have recogni/ed the need to combine multi#le heat sources into a

single system. !his is es#ecially relevant given the groing use of reneable energy in Euro#e. !heir

a##roach, de#icted in igure 1, uses a single large storage tan& as the thermal accumulator in thesystem.

Each heat source de#osits heat to the thermal accumulator henever it o#erates. !he thermal mass ofthe ater acce#ts this heat ithout abru#t changes in tem#erature, and thus hel#s #rotect the heat

source3s4 from short"cycling.

!hermal accumulators service both s#ace heating and domestic ater heat loads. A common

configuration uses an internal heat e(changer near the bottom of the tan& to acce#t in#ut from an array

of solar collectors. Placing the coil there allos the collector circuit to o#erate at the loest #ossibletem#erature and the highest #ossible efficiency.

Another coil, located near the to# of the tan&, is used to add heat from a boiler hen necessary. !his

allos the coil to boost the tem#erature of the ater at the to# of the tan& ithout disturbing thetem#erature stratification ithin the tan& 3e.g., &ee#ing the armest ater at the to# and coolest ater

at the bottom4. !he heat source connected to the u##er coil ould o#erate to maintain a suitable

delivery tem#erature for domestic hot ater. !his conce#t is shon ith a bit more detail in igure 2.

Figure 2

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!he ma5ority of the ater in the thermal accumulator de#icted in igure 2 is #otable ater. An anti"

scald tem#ering valve intercedes beteen this ater and the #lumbing fi(tures. !his is an absolutely

critical detail given the #otential for high tan& tem#eratures during sunny arm eather.

$ther thermal accumulators are designed to hold s#ace heating ater in the main tan& shell. Potable

ater is heated ithin a small internal tan& mounted in the u##er #ortion of the main tan&. !he inner

tan& is com#letely surrounded by hot ater and, thus, has am#le area to ra#idly transfer heat to #otableater it contains 3see igure 64. $#erating modes are the same as shon in igure 2.

Figure 3

Still another variation is an un#ressuri/ed #oly#ro#ylene tan& that sim#ly contains stationary ater as

the mediating thermal mass. All subsystems add or remove heat from this ater via a coil or internaltan&. An e(am#le of such a tan& is shon in igure 7.

!he manufacturer of this #roduct em#hasi/es that the stationary ater ithin the tan& #rovides su#eriortem#erature stratification 3armest ater at the to#, coolest ater at the bottom4.

Another cited advantage of this design is that minimal volumes of domestic ater are stored ithin the+% coil. !his reduces the #otential for legionella groth. 9ee# in mind that a small volume of +%

doesn:t im#ly lo delivery ca#ability. !here:s alays #lenty of thermal energy #ar&ed in the tan&:s

stationary hot ater ready to instantly move through the coil all to &ee# the +% floing.

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Figure 4

!he heat storage ability of this thermal accumulator can be enhanced by adding u# to 77 #ounds of

#araffin a(, hich melts u#on heating to ;; degrees - 3161 degrees 4 and floats on to# of the ater.

As the tan& cools, the a( goes through a #hase change from li

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!he ay see it, if you:re going to build a high"