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Solar Hot-Water Preheating Systems in IESVE

By Birajan Bhandari on Thursday 1 November 2018

Solar Water Heating systems are commonly used to (pre)heat water, using solar energy for the purposes of reducing the building’s heating loads, energy, costs and carbon emissions. There can often be financial incentives available from the local of federal government, or utility provider, for installing a solar hot-water system. Within IESVE software, this can be applied to space-heating hot-water systems and service hot-water systems in ApacheHVAC. In the example below, we are comparing how various solar collector options affect the predicted energy consumption for a small office building, by preheating the feed water of the Domestic Hot Water (DHW) system. 

Figure 1: Preheating the DHW System in ApacheHVAC with Solar Water Preheating 

Note that a hot-water tank is (optionally) included in this analysis, and it is worth mentioning that for some swimming pool applications, which include solar hot water preheating, the “tank” can be considered a proxy for the swimming pool. 
 
Four service hot water systems (or DHW systems) are compared, including three types of solar collectors:

a) No solar preheating
b) Flat Plate Collector (FPC)
c) Evacuated Tube collector (ETC)
d) Parabolic Collectors (PC)


IESVE Model Inputs:

To have a common baseline for all three of the solar collector options, the following assumptions have been made for the small office building:

  • Location = San Diego, California
  • Mass flow rate of municipal feed water = 11.4 gallons/hour / 0.012 litres/second 
  • Temperature of municipal feed water = 60°F / 15.5°C
  • Target temperature for DHW system = 140°F / 60°C
  • Collector area = 100 ft2 / 9.3 m2

Figure 2 below show two examples of solar collector input data, entered into the ApacheHVAC “Solar Water Heater” dialog, for different solar collectors. The various collector parameters can be obtained from the SRCC database (ICC SRCC1 OG-100 certificate).

Figure 2: Input Data for ‘Flat Plate/Evacuated Tube’ Collector (left) and Parabolic Collector (right)

Note, the collector fluid for Flat Plate Collector and Evacuated Tube Collector is assumed to use propylene glycol in order to avoid freezing. Therefore, if the collector fluid is water only, the mass flow rate of the fluid may need to be modified in order to generate the same amount of heat transfer. This can be estimated using the following equation: 

mw  X cpw = mf  X cpf

Where mw and mf are the mass flow rates of water and fluid respectively, and cpw and cpf are the specific heat capacities of water and collector fluid respectively. 

IESVE Model Results: 

HVAC system sizing and whole-building energy simulation processes are compared for each scenario through Apache and the results of the simulation are shown below. 

Figure 3: Energy & Carbon Consumption with Solar Preheating

For this particular situation, there are 64%, 65% and 68% reductions in CO2 emissions for flat plate, evacuated tube and parabolic solar collectors respectively, relative to the baseline case.  

The parabolic solar collector system has a less pronounced reduction of energy during the summer months than the winter months because the parabolic solar collector only ‘collects’ direct irradiation, whereas the flat place and evacuated tube always ‘collect’ both direct and diffused irradiation, and so can be a more appropriate selection in more overcast climates.

One of the more common methods of selecting a solar collector is through selection of the desired outlet temperature from the collector because the installation may be coupled with radiant HVAC systems, swimming pool applications, DHW systems and/or with condensing boilers. All of these perform favorably with low temperature hot water systems, meanwhile a parabolic solar collector may be more applicable to a higher-temperature hot-water heating system. The efficiency of a collector varies relative to the outlet temperature of the collector and this relationship should be considered for the proper selection of solar collector. 

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1 Solar Rating and Certification Corporation: http://solar-rating.org/certification_listing_directory/