High Performance in Low Flow Solar Domestic Hot Water Systems
Date
1997Author
Dayan, Myrna
Publisher
University of Wisconsin-Madison
Metadata
Show full item recordAbstract
Low-flow solar hot water heating systems employ flow rates on the order of 1/5 to 1/10 of the
conventional flow. Low-flow systems are of interest because the reduced flow rate allows
smaller diameter tubing, which is less costly to install. Further, low-flow systems result in
increased tank stratification. Lower collector inlet temperatures are achieved through
stratification and the useful energy produced by the collector is increased.
The disadvantage of low-flow systems is the collector heat removal factor, FR, decreases with
decreasing flow rate. A serpentine collector has the potential to perform better than a
conventional header-riser collector in low-flow systems due to the earlier onset of turbulent flow
which enhances the internal heat transfer coefficient. The onset of turbulent flow is a function of
the tube diameter and flow rate per tube.
Many solar domestic hot water systems require an auxiliary electric source to operate a pump in
order to circulate fluid through the solar collector. A photovoltaic driven pump can be used to
replace the standard electrical pump. PV driven pumps provide an ideal means of controlling
the flow rate, as pumps will only circulate fluid when there is sufficient radiation. The reduction
of parasitic pumping power can also reduce on-peak utility demand. The PV pump, if
adequately designed, decreases the system performance by a negligible amount.
ii
There has been some confusion as to whether optimum flow rates exist in a solar domestic hot
water system utilizing a heat exchanger between the collector and the storage tank, as
commonly employed for freeze protection. It was found that there exists thermal optimum or at
least economical optimum flow rates when it is considered that low flow rates incur less
hydraulic costs. Peak performance was always found to occur when the heat exchanger tank-
side flow rate was approximately equal to the average load flow rate. For low collector-side
flow rates, a small deviation from the optimum flow rate will dramatically effect system
performance. However, system performance is insensitive to flow rate for high collector-side
flow rates.
Antifreeze solutions have temperature dependent properties such as density and specific heat.
The effect of large temperature dependent property variations experienced by ethylene glycol
and propylene glycol affect the optimum flow rate through the collector-side of the heat
exchanger. The increased viscosity of the glycol at low temperatures impedes the onset of
turbulence, which is detrimental to the heat exchanger UA.
Subject
Thesis (M.S.)--University of Wisconsin--Madison, 1997.
Dissertations Academic Mechanical Engineering.
University of Wisconsin--Madison. College of Engineering.
Permanent Link
http://digital.library.wisc.edu/1793/7729Description
Under the supervision of Professors William Beckman and Sanford Klein; 243pp.
Citation
Dayan, M. (1997). High Performance in Low Flow Solar Domestic Hot Water Systems. Master's Thesis, University of Wisconsin-Madison.