Simon J. Rees

Simulation of a domestic ground source heat pump system using a three-dimensional numerical borehole heat exchanger model

Common approaches to the simulation of borehole heat exchangers (BHEs) assume heat transfer in circulating fluid and grout to be in a quasi-steady state and ignore fluctuations in fluid temperature due to transport of the fluid around the loop. However, in domestic ground source heat pump (GSHP) systems, the heat pump and circulating pumps switch on and off during a given hour; therefore, the effect of the thermal mass of the circulating fluid and the dynamics of fluid transport through the loop has important implications for system design. This may also be important in commercial systems that are used intermittently. This article presents transient simulation of a domestic GSHP system with a single BHE using a dynamic three-dimensional (3D) numerical BHE model. The results show that delayed response associated with the transit of fluid along the pipe loop is of some significance in moderating swings in temperature during heat pump operation. In addition, when 3D effects are considered, a lower heat transfer rate is predicted during steady operations. These effects could be important when considering heat exchanger design and system control. The results will be used to develop refined two-dimensional models.

Implementation and Validation of Ground-Source Heat Pump System Models in an Integrated Building and System Simulation Environment

Despite the low energy consumption and lower maintenance benefits of ground-source heat pump (GSHP) systems, little work has been undertaken in detailed analysis and simulation of such systems. Long-term transient ground heat transfer significantly affects the performance of these systems. Annual and multi-year simulation consequently becomes an invaluable tool in the design of such systems—both in terms of calculating annual building loads and long-term ground thermal response. The EnergyPlus program, which makes use of variable time-step sizes in its simulation of building systems, was extended to allow multi-year simulations. Models of a water-source heat pump and a vertical borehole ground-loop heat exchanger have been implemented in Energy-Plus. The ground heat exchanger model uses Eskilsons “g-functions” to model response to time-varying heat fluxes and has been extended to include a computationally efficient variable time-step load aggregation scheme. The performance of this model has been compared with an analytical line source approximation. For a steady periodic input that included pulsated heat extraction, the model agreed with the analytical solution to within 2°C. The heat pump model was able to predict power and heat transfer rates over a wide range of operating conditions to within ±10% of published data. Experimental data from the Oklahoma State Hybrid Ground Source Heat Pump Laboratory have been used to validate both the heat pump and ground heat exchanger models. System simulation results were compared with five days of experimental data. The results showed an average error in the predicted ground heat transfer rate of less than 6% and average errors in the predicted heat pump power and the predicted source-side heat transfer rate of less than 3% and 4%, respectively. Using these models, it is possible to represent GSHP systems in a flexible way and examine their performance over the extended periods required for proper analysis.

A Model for Simulating the Performance of a Pavement Heating System as a Supplemental Heat Rejecter With Closed-Loop Ground-Source Heat Pump Systems

The thermal loads of commercial and institutional buildings are generally cooling-dominated. When such buildings use ground source heat pump systems (GSHP) they reject more heat to the ground-loop heat exchanger than they extract over the annual cycle. In these situations, supplemental heat rejecters can be used to reduce the required size of the ground-loop heat exchanger, thereby reducing the first cost of the system. This paper describes the development, validation, and use of a design and simulation tool for modeling the performance of a hydronic pavement heating system as a supplemental heat rejecter in ground-source heat pump systems. The model uses a finite difference method to solve the transient two-dimensional heat conduction equation and has been formulated for use in component based system simulation. Full-scale experiments were conducted concurrently with the development of the model, the results of which have been used for validation purposes. An example application simulation is presented to demonstrate the use of the model as well as the viability of the use of pavement heating systems as supplemental heat rejecters in GSHP systems.

A nodal model for displacement ventilation and chilled ceiling systems in office spaces

Abstract A nodal model has been developed to represent room heat transfer in displacement ventilation and chilled ceiling systems. The model uses precalculated air flow rates to predict the air-temperature distribution and the division of the cooling load between the ventilation air and the chilled ceiling. The air movements in the plumes and the rest of the room are represented separately using a network of 10 air nodes. The values of the capacity rate parameters are calculated by solving the heat and mass balance equations for each node using measured temperatures as inputs. Correlations between parameter values for a range of cooling loads and supply air flow rates are presented.

A Model for Annual Simulation of Standing Column Well Ground Heat Exchangers

Standing column wells can be used as highly efficient ground heat exchangers in geothermal heat pump systems. This paper describes a computationally efficient numerical model of groundwater flow and heat transfer in and around standing column wells. An approach that utilizes an “enhanced” thermal conductivity to account for the natural groundwater movement, but which explicitly models the induced groundwater flow by “bleed,” is proposed. This model has been validated with experimental data and a reference numerical model (Spitler et al. 2002; Rees et al. 2004; Deng 2004). This simplified model is intended for use in hourly simulation programs or design tools.

Numerical investigation of transient buoyant flow in a room with a displacement ventilation and chilled ceiling system

This paper presents the major findings of the PhD work of Rees, who wrote the paper and is the lead author.

Qualitative Comparison of North American and U.K. Cooling Load Calculation Methods

A qualitative comparison is presented between three current North American and U.K. design cooling load calculation methods. The methods compared are the ASHRAE Heat Balance Method, the Radiant Time Series Method and the Admittance Method, used in the U.K. The methods are compared and contrasted in terms of their overall structure. In order to generate the values of the 24 hourly cooling loads, comparison was also made in terms of the processing of the input data and the solution of the equations required. Specific comparisons are made between the approximations used by the three calculation methods to model some of the principal heat transfer mechanisms. Conclusions are drawn regarding the ability of the simplified methods to correctly predict peak-cooling loads compared to the Heat Balance Method predictions. Comment is also made on the potential for developing similar approaches to cooling load calculation in the U.K. and North America in the future.

Validation of vertical ground heat exchanger design methodologies

This work presents a validation of two common methods for designing vertical ground heat exchangers. Both a simulation-based design tool and the ASHRAE handbook design equation are used to find design lengths for four different real systems, using actual experimental data, including building loads as well as physical parameters as inputs. The measured minimum and maximum ground heat exchanger exiting fluid temperatures were used as the design constraint. The simulation-based design tool predicted the borehole length to within 6% in all cases, while the ASHRAE handbook design equation yielded systems with errors from –21% to 167%. Most of this error can be explained by the way loads are represented in the ASHRAE handbook equation, with differences in the borehole thermal resistance also playing a smaller part. The ASHRAE handbook equation relies on a very simple load representation; although this allows it to be used as a simple hand calculation, it also precludes it achieving acceptable accuracy. It does not appear to be possible to revise the ASHRAE handbook equation so as to both significantly improve its accuracy and allow its use in a simple hand calculation.

Large-eddy simulation of buoyancy-driven natural ventilation in an enclosure with a point heat source

Rising buoyant plumes from a point heat source in a naturally ventilated enclosure have been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and to shed more light on the transitional behaviour of the plume and the coherent structures involved. The Smagorinsky sub-grid scale model is used for the unresolved small-scale turbulence. The Rayleigh number, Ra is chosen to be in the range where spatial transition from laminar to turbulent flow takes place (Ra = 1.5 × 109). The plume properties (source strength and rate of spread) as well as the ventilation properties (stratification height and temperature of stratified layer) estimated using the theory of Linden et al. are found to agree reasonably well with the LES results. The variation of the plume width with height indicates a linear variation of the entrainment coefficient rather than a constant value used by Linden et al. for a fully turbulent thermal plume. Flow visualisation revealed the nature of the large-scale coherent structures involved in the transition to turbulence in the plume. The most excited modes observed in the velocity, pressure and temperature fields spectra correspond to Strouhal number in the range 0.3 ≤ St ≤ 0.55 which is in agreement with those observed by Zhou et al. for a turbulent forced plume. Excited modes less than thisvalue (St = 0.2) were observed and may be due to low-frequency motions felt throughout the flow.

GLAS: engineering a common-user Rayleigh laser guide star for adaptive optics on the William Herschel Telescope

The GLAS (Ground-layer Laser Adaptive-optics System) project is to construct a common-user Rayleigh laser beacon that will work in conjunction with the existing NAOMI adaptive optics system, instruments (near IR imager INGRID, optical integral field spectrograph OASIS, coronagraph OSCA) and infrastructure at the 4.2-m William Herschel Telescope (WHT) on La Palma. The laser guide star system will increase sky coverage available to high-order adaptive optics from ~1% to approaching 100% and will be optimized for scientific exploitation of the OASIS integral-field spectrograph at optical wavelengths. Additionally GLAS will be used in on-sky experiments for the application of laser beacons to ELTs. This paper describes the full range of engineering of the project ranging through the laser launch system, wavefront sensors, computer control, mechanisms, diagnostics, CCD detectors and the safety system. GLAS is a fully funded project, with final design completed and all equipment ordered, including the laser. Integration has started on the WHT and first light is expected summer 2006.