Andrew D. Chiasson

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.

New analytical solution for sizing vertical borehole ground heat exchangers in environments with significant groundwater flow: Parameter estimation from thermal response test data

Accurate prediction of transient subsurface heat transfer is important in sizing ground heat exchangers in ground coupled heat pump systems. This article examines three analytical solutions for the heat transfer characteristics around closed-loop borehole heat exchangers in significant groundwater flow. The first solution is the so-called moving line source solution, the second is based on the groundwater g-function, and the third is a mass transport solution, adapted here using a mass–heat transport analogy. The ground thermal conductivity, groundwater velocity, and borehole thermal resistance are estimated using a parameter estimation technique in conjunction with the analytical solutions and thermal response test data from two sites in close proximity, one with significant groundwater flow, and the other without. The main difference between the mass–heat transport analogy and the moving line source and groundwater g-function solutions is that the mass–heat transport analogy can account for the effects of thermal dispersion.The mass–heat transport analogy yields a favorable comparison to field test data with a very high groundwater flow rate, while the other solutions do not produce a realistic comparison, implying that thermal dispersion is an important parameter in subsurface heat transfer, at least in situations with relatively high groundwater flow rates.

MODELING APPROACH TO DESIGN OF A GROUND-SOURCE HEAT PUMP BRIDGE DECK HEATING SYSTEM

The approach taken to design a hydronic snow-melting system for a bridge deck on an Interstate highway in Oklahoma is described. A vertical borehole, closed-loop ground-source heat pump system is to be used as an energy-efficient means to provide the heating requirement. It is proposed to use the bridge deck as a solar collector in the summer months so that the ground can be thermally “recharged” by circulating fluid from the bridge deck to the ground. A design of this type involves the determination of a number of interdependent variables that are difficult or impossible to accurately quantify by conventional design practices. Therefore, mathematical models of a hydronically heated pavement slab, water-to-water heat pump, and a ground loop heat exchanger were developed for use in a detailed system simulation. The simulation allowed design parameters such as minimum heat pump entering fluid temperature, number of heat pumps, and flow rate to be varied, and several potential designs that kept the bridge surface temperature above freezing were selected. The hourly heating loads for candidate configurations were then used to estimate the required size of the ground loop heat exchanger. There is a trade-off between the number of boreholes that make up the ground loop heat exchanger and the number of heat pumps required. Of the configurations examined, a system with 16 heat pumps of nominal 106-kW capacity and 250 boreholes, each 76 m deep, was selected.

Evaluation of Electricity Generation From Underground Coal Fires and Waste Banks

A temperature response factors model of vertical thermal energy extraction boreholes is presented to evaluate electricity generation from underground coal fires and waste banks. Sensitivity and life-cycle cost analyses are conducted to assess the impact of system parameters on the production of 1 MW of electrical power using a theoretical binary-cycle power plant. Sensitivity analyses indicate that the average underground temperature has the greatest impact on the exiting fluid temperatures from the ground followed by fluid flow rate and ground thermal conductivity. System simulations show that a binary-cycle power plant may be economically feasible at ground temperatures as low as 190 {sup o}C.

Horizontal Ground Heat Exchangers

This study aims to present a comparison between temperature measurements relative to horizontal ground heat exchangers with predicted values using thermal response test. A scale 1 test facility of horizontal ground heat exchangers has been implemented in BRGM (Orleans France) to test performances in real conditions. The heat exchanger is divided in four parts of 100 m2 each with different characteristics: a sunny grass a shaded grass a sunny car-park a shaded car-park These different configurations have been chosen to compare the performances of ground heat exchangers in different environments (surface state, boundary conditions). Furthermore, the temperature in the soil is measured continuously at 3 different levels (-0.5 m, -1 m, -1.5 m). To cartography the temperature field at these 3 depths, optical fibers are distributed in the underground and the use of a distributed temperature sensor (DTS) allows to accurately measure the temperature in the soil surrounding the ground heat exchanger. A thermal response is carried out on the horizontal ground heat exchanger. A monitored constant heating power is injected at a constant mass flow. The temperature measurements at the 3 different levels of a threedimensional numerical model of ground heat exchanger. The thermal response test gives in particular the noticeable differences between the 4 different conditions of each part of the ground heat exchanger. The conclusion will give indications on the consequences of uncertainty about soil thermal properties on sizing.

15 – Waste heat rejection methods in geothermal power generation

This chapter focuses on the theoretical and practical considerations of the methods of heat rejection to the environment from geothermal power plants. The influence of heat exchanger effectiveness of the condenser and the heat rejection equipment on the thermodynamic efficiency of the power station is discussed. Generalities are made to compactly review the numerous methods of heat rejection from geothermal power plants. Condenser configurations, heat transfer characteristics, and operating and maintenance issues of each method are discussed. Condensers used in geothermal power plants are mainly of the direct-contact or surface type, with methods of heat dissipation to the environment generally consisting of wet cooling towers, air-cooled condensers, or a combination of wet-dry (or evaporative) condensers. The actual choice depends on designer preference and what the geothermal site characteristics dictate.