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Abstract
Ground-coupled heat pumps are increasingly being utilized to heat and cool buildings. Although it is difficult to size and to predict their behavior and performance, their design can be optimized via simulations. EnergyPlus is a popular energy simulation program for modeling building heating and other energy flows and, since it is organized to consider borehole heat exchangers via the well-known g-functions approach, it can be used advantageously for that purpose. The Capacity Resistance Model is another recent numerical simulation tool devoted to ground and borehole heat exchangers. In this work, two methods to calculate the g-fucntions were analyzed, using as case-study a real office building, whose imbalance between the heat extracted and injected into the ground was found to be appreciable. The energy imbalance involves a ground temperature drift affecting the system efficiency. The results of the EnergyPlus g-functions and the Capacity Resistance Model model approaches were compared. The capacity of the two methodologies to accurately simulate this phenomenon were analysed also with reference to the available building's long-term monitoring data. The analysis showed the importance of using g-functions suitable to reflect the layout of the borehole field, in order to correctly evaluate the energy performance of the entire ground source heat pump system.
| Nomenclature | ||
| a | = | thermal diffusivity (m2/s), surface absorptance (-) |
| C | = | volume heat capacity (J/K) |
| COP | = | coefficient of performance in heating mode (-) |
| EER | = | coefficient of performance in cooling mode (-) |
| hext | = | fonvection heat transfer coefficient at ground surface (W/(m2 K)) |
| i | = | ground discretization index in radial direction |
| j | = | ground discretization index in vertical direction |
| L | = | length (m) |
| Lbore | = | borehole length (m) |
| q | = | specific heat load (W/m) |
| Q | = | heat rate (W) |
| P | = | power (W) |
| rmax | = | radius from axis borehole beyond which the undisturbed ground is considered (m) |
| R | = | thermal resistance (K/W) |
| Rext | = | convection thermal resistance at ground surface per unit area (m2 K/W) |
| Rp0 | = | thermal resistance between the pipe and borehole wall (m K/W) |
| RppA | = | thermal resistance between adjacent pipes (m K/W) |
| RppB | = | thermal resistance between opposite pipes (m K/W) |
| SCOP | = | seasonal coefficient of performance in heating mode (-) |
| SEER | = | seasonal coefficient of performance in cooling mode (-) |
| T | = | temperature (K) |
| Text | = | external air temperature (K) |
| Tg | = | undisturbed ground temperature (K) |
| Tsky | = | sky temperature (K) |
| z | = | depth (m) |
| Greek symbols | ||
| ϵ | = | surface emittance (-) |
| λ | = | thermal conductivity (W/(m K)) |
| τ | = | time (s) |
| Δz | = | length of control volume in vertical direction (m) |
| Subscripts | ||
| b | = | borehole, borehole zone, building |
| d | = | deep zone |
| c | = | cooling |
| el | = | electrical |
| f | = | fluid |
| g | = | ground |
| h | = | heating |
| hp | = | heat pump |
| i | = | inside |
| in | = | inlet |
| L | = | load side |
| nom | = | nominal |
| out | = | outlet |
| r | = | radial direction |
| s | = | surface zone |
| S | = | source side |
| z | = | depth direction |
Acknowledgments
The authors are grateful to M. Cimmino of the Ecole Polytechnique de Montréal (Canada) and to J.D. Spitler of Oklahoma State University (USA) for making the g-function softwares used in this work available to us. Special thanks are due to HiRef S.p.A. for providing the full set of performance data concerning the water to water heat pump examined in this work.