WP1-2 Hydraulic concepts, control strategy


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Technical University of Denmark (DTU)


Federico Bava




Simon Furbo (DTU)

Søren Elisiussen (Arcon-Sunmark A/S)

Short description

The project is focused on solar collector fields for solar heating plants in district heating systems. Detailed simulation models for different solar collector types and differently designed and controlled solar collector fields consisting of a number of collector rows with collectors connected in series in each row will be developed and validated by means of measurements from the solar collector test facility at the Technical University of Denmark and from Danish solar heating plants.

The solar collector field can include both collectors without and with polymer foil used as convection barrier to reduce heat losses. The flow distribution in the collector field is being investigated. Presently, the flow distribution in Danish solar collector fields is determined by mechanical balancing valves installed in each collector row and set in such a way that a homogeneous temperature is obtained at the end of each row in nominal operating conditions. Though, this approach does not consider how the flow distribution is when the field is operated in different conditions of total flow rate and solar irradiance.

Other aspects which are considered are the shadow effect from one row to the following ones, heat loss of the pipes in the solar collector loop and heat capacity of the solar collectors as well as the collector efficiency of different collector types for different collector tilts and different flow rates.

Calculations of the thermal performance of differently designed collector fields with different control strategies will be carried out with different weather data in order to optimize the collector field hydraulic and the control strategy for different district heating systems.

During the PhD study three months were spent at the company Arcon-Sunmark A/S, to learn directly from people who have wide experience in designing, planning and installing large solar collector fields. In fact, Arcon-Sunmark is the world's leading manufacturer of large-scale solar thermal plants for the district heating sector and others, installing most of the Danish solar collector fields since 1974.

Solar heating plants in district heating systems are installed in large numbers in Denmark: the solar collector area of these installations was approximately 900,000 m2 at the end of 2015, making the country the front-runner in this field. The area of solar collector fields is expected to increase in the next years. In a scenario characterized by a strong growth in the installed solar collectors' capacity, even small efficiency improvement may lead to a large increase in the overall energy production in absolute terms. For this reason optimization in the solar field design and control strategy plays a key role.

Today solar collector fields are designed and controlled based on the experience of solar collector manufacturers and consultants and on simple optimization tools. In order to improve the basis for design and control of optimized solar collector fields there is a need for detailed simulation models which can be used to estimate the thermal performance of a collector field, in different operating conditions.
During this PhD project, the flow distribution in the collector field, which can affect significantly the field performance, is carefully investigated. Additionally, shadow effect, heat losses from the pipes in the solar collector loop and the heat capacity of the solar collectors are considered too. Moreover, the influence of different flow rate and tilt angle on the collector efficiency will be taken into account. The developed models will be validated against experimental measurements carried out at Technical University of Denmark and data from Danish solar heating plants. The validated models will be used to optimize the field hydraulic and control strategy (Figure 1).

The numerical models simulating the entire solar collector fields are developed in TRNSYS, given the wide use of this software in the sector of solar thermal energy.

The main developed model will be validated against measured data coming from the Danish solar collector field in Høje Tastrup (Denmark) and then used to optimize the control strategy and the layout of the collector field. The collector field in Høje Tastrup was installed by Arcon-Sunmark at the end of 2014. Beside the already installed measuring equipment, additional pyranometers were mounted, in order to monitor both the total and the diffuse radiation in different parts of the collector field.

In order to develop the simulation model to study the flow distribution in the collector field, the pressure drop across a number of components needs to be evaluated, first of all that across a single solar collector. For this specific purpose, a Matlab model was developed to evaluate the pressure drop in different flow conditions (Figure 2). The model was validated against measurements carried out on a large Arcon collector at the Technical University of Denmark (DTU).

As TRNSYS is unable to evaluate flow distribution in parallel connections, a Matlab code has been developed to evaluate the flow distribution within the collector array and pass the results to the main TRNSYS model (Figure 3).

Additionally, the influence of flow rate, tilt angle and foil convection barrier on the collector efficiency is investigated through experimental tests carried out at DTU and compared with results of the solar collector simulation software Soleff, developed at DTU.

Experimental set-up:

A test facility consisting of two large ARCON solar collectors, commonly used in Danish solar collector fields, was installed at DTU and used to carry out efficiency tests in different operating conditions of flow rate, collector tilt angle and fluid type.

Two the collectors are identical with the only difference being a fluorinated ethylene propylene (FEP) foil between glass cover and absorber, acting as a convection barrier to reduce the heat losses (Figure 4). Additionally, two plastic pipes were connected to the inlet and outlet of the collector and then to a differential pressure sensor, in order to measure the pressure drop across the collector in different operating conditions (Figure 5). The fluid flow rates were measured by two Kamstrup electromagnetic flow meters (models MP240 and MP115). The inlet temperatures are measured by type TT thermocouples using a copper-constantan junction, while the temperature differences between outlet and inlet temperature were measured by thermopiles with five copper-constantan junctions at each measuring point. The total and diffuse radiation on the collector plane was measured by CM11 pyranometers, produced by Kipp & Zonen and classified as secondary standard.

A second experimental setup will consist of the solar heating field at Høje Tastrup, where, beside the measuring equipment already installed by the manufacturer, additional instrumentation, such Kipp&Zonen CM11 pyranometers, were installed to accurately measure both total and diffuse radiation in different parts of the field.

The model evaluating the field flow distribution will be validated against measurements. In fact, the flow rate in the different collector rows can be indirectly measured by measuring the pressure drop across each balancing valve installed at the row inlet.

Numerical tools:

The main simulation model is developed in TRNSYS, which was already used for a study on optimization of solar collector row composition presented at the SHC 2014 conference in Beijing.

However, as TRNSYS is unable to evaluate flow distribution in parallel connections, a Matlab code was written to evaluate the flow distribution in the collector field. The model is based on literature correlations and self-developed equations, such as those for the solar collectors, which are derived from another Matlab model, which was validated against measurements carried out at the DTU collector test facility.

Working plan and current status

  • Literature study: always ongoing
  • Testing of solar collectors with and without polymer foil: completed
  • Study on optimization of solar collector row composition: completed
  • Matlab model for pressure drop in solar collector: completed
  • Validation of pressure drop model: completed
  • Matlab model for field flow distribution: completed (minor changes are still needed)
  • TRNSYS model of solar collector field: ongoing
  • Validation of TRNSYS model: to be done
  • Modification of TRNSYS Type 539 to take into account flow rate effect on collector efficiency: ongoing
  • Study of control strategy: ongoing
  • Optimization of control strategy and field layout: to be done