Overview of Solutions for the Low-Temperature Operation of Domestic Hot-Water Systems with a Circulation Loop

Theofanis Benakopoulos,Robbe Salenbien,Michele Tunzi,William Vergo,Svend Svendsen

doi:10.3390/en14113350

Theofanis Benakopoulos, Robbe Salenbien + Show 3 more

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https://doi.org/10.3390/en14113350

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Abstract

The operation of typical domestic hot water (DHW) systems with a storage tank and circulation loop, according to the regulations for hygiene and comfort, results in a significant heat demand at high operating temperatures that leads to high return temperatures to the district heating system. This article presents the potential for the low-temperature operation of new DHW solutions based on energy balance calculations and some tests in real buildings. The main results are three recommended solutions depending on combinations of the following three criteria: district heating supply temperature, relative circulation heat loss due to the use of hot water, and the existence of a low-temperature space heating system. The first solution, based on a heating power limitation in DHW tanks, with a safety functionality, may secure the required DHW temperature at all times, resulting in the limited heating power of the tank, extended reheating periods, and a DH return temperature of below 30 °C. The second solution, based on the redirection of the return flow from the DHW system to the low-temperature space heating system, can cool the return temperature to the level of the space heating system return temperature below 35 °C. The third solution, based on the use of a micro-booster heat pump system, can deliver circulation heat loss and result in a low return temperature below 35 °C. These solutions can help in the transition to low-temperature district heating.

Highlights

Test Results of domestic hot water (DHW) Tank with a Heating Power Limitation Functionality and a Safety Function for Peak Loads In Figure 12, the demonstration of the heating power limitation functionality, without the additional safety functionality under the EnergyLab Nordhavn project [34] during 31 December 2018, showed that during days with high demand, such as New Year’s Eve, the temperature of the tank drops below the acceptable temperature for comfort
The main result is the recommendation of three solutions depending on combinations of the following three criteria: district heating supply temperature, relative circulation heat loss due to the use of hot water, and the existence of a low-temperature space heating system
For typical DHW systems with a storage tank, high district heating (DH) supply temperature, and moderate circulation heat losses, the solution based on heating power limitation with a safety functionality may secure the required DHW temperature at all times, resulting in the limited heating power of the tank, extended reheating periods, and a DH return temperature of below 30 ◦C

Summary

Introduction

Low-temperature district heating (LTDH) systems, with supply and return temperatures of approximately 55 and 25 ◦C, respectively, offer an economical solution to realize a fossil-free energy system [4,5,6,7,8]. LTDH systems can only be realized by securing the low-temperature operation of the existing buildings heated by DH [6]. There is great potential for the low-temperature operation of space heating in existing buildings [6,7]. These systems have been designed for ambient conditions that rarely occur [9,10,11,12,13,14]. The design condition can be characterized as conservative for the current actual operating conditions

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