Exergetically optimized operation of building heating and cooling system using a dynamic control system and flexible integration of a fully monitored geothermal field

Sensorequipment within geothermal shaft Copyright: EBC Automatic drives and volume flow sensors within the south shaft of the E.ON ERC main building’s borehole heat exchanger field

One challenge in modern construction is to reduce energy and heating costs. An exergetically optimal energy use within a low-temperature energy system can approach this issue. We, GGE and EBC, cooperate towards the sustainable integration of the E.ON ERC main building’s geothermal field into its energy system while trying to adjust energy flows and energy use as efficient as possible. More precisely, the aim of GGE in this project is to establish a 3-D subsurface model for the long-term prediction of geothermal-energy use. EBC is aiming towards a control strategy for the building energy system with a minimal exergy loss using GGE’s geothermal predictions. We will achieve this via an integrated approach for the geothermal field and the energy conversion system. The German Federal Ministry of Economic Affairs and Energy funds this project.

Due to limited fossil and economic resources, we have to achieve a cost-benefit optimal and energy efficient operation of buildings. At the E.ON ERC main building, the only way of achieving this is a sustainable operation of its geothermal field. The operation parameters of geothermal fields react sensitive to overuse, whereas shifting too small energy amounts is not efficient. In other words, the potential for cooling or heating of the field decreases when the system dissipates too much energy into the field or when it extracts too much energy from the field respectively. Thus, in the end, one must keep the field’s energy balance within certain ranges. From an economical point of view, the immense costs of geothermal field installations aggravate the issue of efficient and sustainable operation, since the field has to keep its relatively low operation cost throughout the buildings lifetime. This is only possible by a balanced field operation.

The heating and cooling system of the E.ON ERC main building uses an interaction of different low-temperature sources. Besides the emitted heat of server rooms and cooled-down areas like computer rooms for students within the building’s core, the building uses the geothermal temperature gradient from the underground as a heat source during winter and heat sink during summer. In our case, a geothermal field of, consisting of 40 borehole-heat-exchanger (BHE), provides access to the underground energy. The field is located around the E.ON ERC main building. Each BHE is 100 meter deep and a 35-%-ethylene-glycol brine fluid prevents frost damage.

Four main ingredients drive the research project‘s approach. First, the possibility of a dynamic field operation, meaning a variable volume flow for each single BHE of the geothermal field, to adjust the extracted or dissipated amount of energy for each BHE individually. Second, detailed sensor measurements of the field and building operation. Third, system and building models, in order to calculate and predict the forthcoming energy needs and the consequences of different operation parameters. Fourth, a control strategy that is able to vary the geothermal field’s integration.

In order to provide a dynamic field integration, we implement actuators that allow for an individual operation of each BHE. For each BHE, automatic ball valves offer the opportunity to adjust the volume flow. These actuators, together with a volume flow sensor in each BHE offer the possibility to control the volume flow automatically for each BHE. Temperature sensors in inlet and outlet make an energy-oriented control possible. Further control regimes account for friction-optimal operation and for uniform volume flow operation, meaning the same volume flow for each BHE. A controlled reduction of the brine pump’s speed with the maximal opening angels of BHEs’ valves provides this friction-optimal operation. We equipped the brine pumps with a Building Automation and Controls Network (BACnet) interface, offering remote control for several further pump control strategies, e.g. speed adjustment, pressure adjustment or temperature-difference control. A central magnetic-inductive volume flow sensor measures the sum of the volume flows through each BHE. The building management system integrates the field automation and control, thus, we can use different interfaces to command set points or switch field operation modes. Further, the building management system stores all measurement data into two SQL databases. Refer to [1] for further information about the monitoring, control and interface system.

We need informed calculations and decisions in order to conduct simulations and in order to develop potential control strategies. On the field’s side, different kinds of measurement equipment provide this information. A Distributed Temperature Sensing system provides both, temperature data of the geothermal field for a long-term monitoring and temperature data within the building. This is an optical fiber measuring system. Based on the Raman Effect temperature within a fiber can be calculated from temperature depends changes of the refraction index. The equipped BHE provides monitored temperatures of the entire underground at the BHEs’ positions. On the other hand, temperature sensor rings within one single BHE provides information on the soil’s temperature distribution in the vicinity of this BHE and, thus, conclusions towards possible groundwater flow strength and direction. This allows an integration of groundwater flow parameters into numerical simulation of the geothermal field. On the building side, magnetic-inductive flow sensors, additional temperature and humidity sensors, as well as all the available sensors from the building automation system form an extensive monitoring system that allows for interpretation and analysis of the building operation.

Numerical calculations are one key tool in the project. We use simulations to predict the influences of different control strategies towards energy efficiency. We investigate different operation modes for the geothermal field for a sustainable long-term usage of the heat exchanger field. In this context, different modes mean the time based adaptation of cooling and heating phases or the temporary turning-off of the geothermal fields’ sectors. They base on a model of the soil in an area of 140 80 meters and a depth of 130 meters. SHEMAT, a simulation tool for coupled flow, heat transfer, transport and chemical water-rock interaction carries out the simulations for the geothermal field. Using the temperature difference between inlet and outlet temperatures of the BHE field as input parameters, this finite difference code allows the computation of long-term impact on the ground caused by the BHEs. This again sets more-precise boundary conditions for the heating and cooling operation. On the building side, we use Modelica and Simulink to carry out simulation of the physical components and for the control logics respectively. These simulations aim for a better understanding of the systems behavior and for a simulation-based control. Thereby, we are able to shift parameters to adjust the energy transferred with the BHE field. The geothermal numerical model has to estimate a nearly optimum-operation parameter regime in order to fulfill the buildings needs while respecting the constraints for a sustainable long-term operation.

In order to react to varying energy transfer rates for the field, we implement a sophisticated control strategy into the building, which provides the possibility to adjust operation times and intensities of the geothermal field usage. For example, the geothermal free-cooling mode is block-able—in order to decrease operation time; further, a variation of the integration temperature of the field is possible.

The outcomes of the project are in demonstrating an innovative integrated operation methodology, in advanced geology, building and system related simulation models, in novel control building control strategies, as well as in advances in communication interfaces and technologies between the different instances. A future research prospect on EBC’s side is to have an integrated simulation that allow for evaluation of different control strategies and a conceptualization of the modus-based control strategy.

We grateful acknowledge the financial support of the German Federal Ministry of Economic Affairs and Energy (BMWi), promotional reference 03ET1022A.

References:

[1] Futterer J, Constantin A, Schmidt M, Streblow R, Muller D, Kosmatopoulos E. A multifunctional demonstration bench for advanced control research in buildings—Monitoring, control, and interface system. In: Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE; 2013. p. 5696–701 Available from: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6700068.