S. Furbo1* and J. Fan1

1 Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118,

DK-2800 Kgs. Lyngby, Denmark
* Corresponding Author, sf@byg. dtu. dk
Abstract

Side-by-side tests of two small low flow SDHW systems based on mantle tanks have been carried out in a laboratory test facility. The systems are identical with exception of the circulation pumps and the solar collector loops. One of the systems is equipped with a low flow SOLAR 15-65 pump from Grundfos A/S and a so-called “lifeline” solar collector loop based on a %” copper pipe for the hot solar collector fluid pumped from the solar collector to the mantle tank and on a 5/16” copper pipe for the cold solar collector fluid pumped from the mantle tank to the solar collector. Both the hot and the cold pipes are insulated with glass fibre and enclosed by a PVC jacket with a diameter of 38 mm. The other system is equipped with a normal circulation pump for solar heating systems, SOLAR 15-40 from Grundfos A/S and a normal solar collector loop consisting of 10/8 mm copper pipes insulated with 9 mm INSUL-TUBE insulation.

The tests were carried out with the same daily hot water consumption of 100 l/day. Measurements of the thermal performance of the systems, the volume flow rates in the solar collector loops and the energy consumption of the pumps have been carried out for a measurement period of about 4 months. The measurements show that the thermal performance of the SDHW system can be increased by about 1% by replacing the normal solar collector loop with the lifeline. The extra pump energy for the system with the lifeline and the SOLAR 15-65 pump was in the measurement period higher than the extra thermal performance for the system with the lifeline and the SOLAR 15-65 pump. Consequently, the “lifeline approach” is not justified from a thermal performance point of view.

The laboratory measurements will be continued with an improved solar collector loop for the SDHW system with the SOLAR 15-65 pump. The lifeline was in July 2008 replaced with small well insulated separate copper pipes. Further measurements will elucidate if the thermal advantage by using such small pipes is large enough to compensate for the extra pump energy required by the SOLAR 15-65 pump.

Keywords: SDHW systems, low flow, laboratory test, circulation pumps, pump energy, solar collector loops

1. Introduction

Small SDHW systems can with advantage be designed as schematically shown in figure 1.

Low flow SDHW systems based on vertical mantle tanks have a number of advantages compared to similar high flow SDHW systems based on vertical hot water tanks with a built in heat exchanger spiral [1]:

• The yearly thermal performance is increased by 10%-25% depending on the solar fraction.

• Lime deposits in the mantle tank are a factor of 2.5 lower than the lime deposits in a similar hot water tank with a built in heat exchanger spiral.

• Small pipe dimensions can be used in the solar collector loop. The installation can therefore be facilitated. For instance, small all-in-one solutions with two small pipes, pipe insulation and a wire for the control system can be used as a solar collector loop resulting in decreased cost of the solar heating system.

Further, if small pipes are used in the solar collector loop, the heat loss of the pipes can be reduced and the thermal performance of the system can be increased. In order to make use of small pipes in the solar collector loop, high pressure pumps are needed. Grundfos A/S has recently introduced such circulation pumps for solar heating systems on the market, for instance the SOLAR 15-65 pump. In this paper it is for small SDHW systems experimentally investigated how much the thermal performance is increased and how much the pump energy is increased by making use of small pipes and a high pressure pump in the solar collector loop.