Energy pile field simulation in large buildings : Validation of surface boundary assumptions

As the energy efficiency demands for future buildings become increasingly stringent, preliminary assessments of energy consumption are mandatory. These are possible only through numerical simulations, whose reliability crucially depends on boundary conditions. We therefore investigate their role in numerical estimates for the usage of geothermal energy, performing annual simulations of transient heat transfer for a building employing a geothermal heat pump plant and energy piles. Starting from actual measurements, we solve the heat equations in 2D and 3D using COMSOL Multiphysics and IDA-ICE, discovering a negligible impact of the multiregional ground surface boundary conditions. Moreover, we verify that the thermal mass of the soil medium induces a small vertical temperature gradient on the piles surface. We also find a roughly constant temperature on each horizontal cross-section, with nearly identical average values when either integrated over the full plane or evaluated at one single point. Calculating the yearly heating need for an entire building, we then show that the chosen upper boundary condition affects the energy balance dramatically. Using directly the pipes’ outlet temperature induces a 54% overestimation of the heat flux, while the exact ground surface temperature above the piles reduces the error to 0.03%.Peer reviewe

Modeling an alternate operational ground source heat pump for combined space heating and domestic hot water power sizing

This study developed an alternate operational control system for ground source heat pumps (GSHP), which was applied to determine combined space heating and domestic hot water (DHW) power equations at design temperature. A domestic GSHP with an alternate control system was implemented in a whole building simulation model following the heat deficiency for space heating based on degree minute counting. A simulated GSHP system with 200 L storage tank resulted in 13%-26% power reduction compared to the calculation of the same system with existing European standards, which required separate space heating and DHW power calculation. The periodic operation utilized the thermal mass of the building with the same effect in the case of light and heavy-weight building because of the very short cycle of 30 min. Room temperatures dropped during the DHW heating cycle but kept within comfort range. The developed equations predict the total power as a function of occupancy, peak and average DHW consumption with variations of 0%-2.2% compared to the simulated results. DHW heating added the total power in modern low energy buildings by 21%-41% and 13%-26% at design temperatures of -15 °C and -26 °C, respectively. Internal heat gains reduced the power so that the reduction effect compensated the effect of DHW heating in the case of a house occupied by three people. The equations could be used for power sizing of any heat pump types, which has alternate operation principle and hydronic heating system.Peer reviewe

A tabulated sizing method for the early stage design of geothermal energy piles including thermal storage

| openaire: EC/H2020/856602/EU//FINEST TWINSGeothermal systems are often employed for both the heating and cooling of sustainable constructions. Energy piles (U-shaped heat exchangers inserted into the foundation piles) are widely included in these installations, whose performance is usually estimated by means of complex, time-consuming simulations already at an early design stage. Here we propose a simple methodology, where a hand calculation tool provides the condenser yield per pile meter, ground area yield and demand covered by the heat pump by specifying only building heat load and geometric characteristics of the energy piles field. Our tool is tested by assuming 20 years of operation in a hall-type commercial building in a cold climate. A validated IDA-ICE parametric study couples the heat pump evaporator operation with heat transfer processes between energy piles and soil. Various system configurations are considered and thermal storage in the soil is included. We find that the expected yield is not directly proportional to pile separation, while a smaller extraction power is favoured. Thermal storage in the soil is also confirmed to be critical. Besides our specific quantitative results, our practical guideline is qualitatively general and can be extended to any given building type and climate.Peer reviewe

Residential buildings with heat pumps peak power reduction with high performance insulation

Revised EPBD directive has set ambitious targets for nearly zero energy buildings. In residential buildings, energy performance can be improved mainly by applying better insulation of building fabric and by efficient energy sources, i.e. heat pumps. Electricity use and peak powers will increase when heat pumps, both air to water and ground source heat pumps, are used for heat source in new residential buildings compared to heating solutions that do not use electricity. The purpose of this study was to determine how much the high performance thermal insulation can compensate the increase of electricity use and peak power caused by extensive application of heat pumps in Finland residential buildings. The present study used five residential buildings that describe residential newbuild market. Finnish regulation defines minimum insulation level and high performance insulation level which were applied to single family houses, terraced house and apartment buildings to simulate electric power values all year round. Hourly electrical power values were simulated with dynamic simulation software IDA ICE. Results show that electricity use and peak powers are rising significantly when heat pumps are used, but better insulation level significantly decreases or even fully compensates the amount of additional electric power. The results can be used for the assesment of implications of extensive use of heat pumps to power grid.Peer reviewe

Heat Recovery from Exhaust Air as a Thermal Storage Energy Source for Geothermal Energy Piles

In pursuit of EU directive 2010/31/EU energy performance targets towards design of nearly zero-energy buildings consideration of renewable energy sources in the design is expected. Application of ground-source heat pump (GSHP) and energy piles in cold climate conditions for utilization of renewable geothermal energy may results in GSHP plant high seasonal coefficient of performance (SCOP) as long as source of thermal storage is considered in plant design. This numerical study investigates exhaust air of air handling unit (AHU) as a source of thermal storage for geothermal plant with energy piles, that can be utilized via air-to-liquid heat exchanger installed at the exhaust side of AHU after the rotor heat exchanger and exhaust fan. Modelling is performed in dynamic whole year simulation environment Equa IDA-ICE, where reference commercial hall-type building model is coupled with detailed custom heat pump plant. Exhaust air thermal storage capacity in multiple energy piles field configurations with varying soil conditions, distance between piles and pile lengths is analyzed. Results revealed that exhaust air thermal storage appears to be highly cost effective solution. Graphical figures presented in this paper can be further applied for preliminary exhaust air thermal storage capacity assessment in buildings with energy piles.Peer reviewe

Measured and simulated energy performance of OLK NZEB with heat pump and energy piles in Hämeenlinna

In this work, measured energy use of the building space heating, ventilation supply air heating, appliances and lighting is compared against simulated energy use modelled in IDA ICE. As built energy need and detailed measured input data is applied in building model calibration procedure. Calibrated building model energy performance is studied in both measured and test reference year climate conditions. Previously modelled as built plant automation and implemented control logics are compared against measured. Geothermal plant in this study consists of heat pump, solar collectors, boreholes and energy piles. Heat pump SCOP estimated by post processing according to heat pump manufacturer's performance map is compared against measured SCOP on the monthly basis. Opinion on actual plant operation is given and energy performance improvement potential is quantified. Important parameters for successful building model calibration are presented. Building compliance with Finland NZEB requirements are assessed. The results show good match with measured energy use after the model calibration.Peer reviewe

Assessment of Retrofit Measures for Industrial Halls

The need for renovation is driven mostly by high energy prices and by the amortization of building envelope structures and building systems. Industrial buildings built before 1990s are usually poorly insulated and without ventilation heat recovery. When renovation is considered, it raises a question of what and to which extent to renovate, to achieve the optimal result for the investment. The current study focuses on the building envelope insulation and ventilation renovation options for hall-type industrial buildings. We have analyzed the impact of different renovation measures, regarding envelope insulation and ventilation systems, to the energy consumption and renovation budget for three typical buildings built between the 1960s and 1990s in Finland. The energy consumption calculations have been conducted with building energy and indoor climate simulation tool IDA-ICE. For the economic calculations, we have used the current Finnish energy prices and interest rates with moderate trends for the next 20 yearsto estimate the internal rate of return and net present value of the retrofit measures. The calculations have been done for two building sizes: a large hall, 137m x 66m, and for a smaller hall, 40m x 22m, both with an average height of 8m. The results show, that retrofitting the building envelope only for energy efficiency might not be beneficial when considering a payback period less than 20 years in the case of both large and small hall buildings. In case of smaller halls, some combinations of envelope retrofit can be also economically reasonable. Renovating the ventilation system by applying heat recovery and replacing lighting for energy efficient LEDs would be beneficial for all the initial building cases. Combination of supporting measures as a renovation package would be the most recommendable solution.Peer reviewe

Thermal mass and energy recovery utilization for peak load reduction

Highly energy performing buildings need cost effective solutions which can deliver specified indoor climate and energy performance targets. In this study temperature variation of indoor climate category II according to EN 15251 standard is applied with the aim to allow free floating temperatures in this range to activate internal thermal mass of walls. Main hypotheses are that interior thermal mass of enough thick concrete layers can enable utilization of solar and internal gains resulting in significantly reduced peak loads for both heating and cooling, and reduced overall energy need. In this study, dynamic energy simulations are conducted to identify optimal solutions for a planned experimental building. Impact of energy recovery system on annual heating/cooling need and interior thermal mass on cooling design load are studied. Proposed energy recovery system consists of a piping layer installed into internal layer of a wall or floor structure and coupled with storage tank via piping and circulation pump. This system operates only when specified temperature differences exist that is expected to store excess room heat or cool within accepted indoor temperature range and to distribute it into other building zones. Modelling is performed in dynamic whole year simulation environment IDA-ICE, where a simplified two-zone model of a single-family house along with energy recovery system are modelled. Zones envelope and interior structures are modelled with finite difference wall/floor model accounting for thermal capacitances of structures material layers and exposure to solar radiation passing through detailed window model. Model of a piping layer connected to finite difference wall or floor structure computes heat transfer using logarithmic temperature difference. Rest of the energy utilization system is modelled using IDA-ICE standard model library components. Results reveal that interior thermal mass has significant impact on peak loads and energy need reductions. Modelled energy recovery system is capable of significantly reducing heating need as long as high system flow is maintained.Peer reviewe