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Article Excerpt
INTRODUCTION
With rapid growth of the geo-exchange industry in North America, there has been a growing need for a method to guide rational, suitable, and appropriate applications of geo-exchange technology with the owner's interests in mind. The geo-exchange suitability assessment (GSA) method presented herein was developed to provide a rational framework to assist owners, developers, and industry professionals in making decisions on how best to apply geo-exchange technology, from the initial concept to the start of design. This method is presented as a model of recommended practice for evaluating the overall suitability of a site and highlighting the ground coupling options that best fit site characteristics. The method will be most applicable to commercial, institutional, or larger multiunit systems where engineering practice is warranted. However, the approach of objectively considering project and owner needs, characterizing a site, and letting site features govern the appropriate selection of ground coupling should be applied to any geo-exchange project.
Previous Work on GSA
A number of previous studies relate to site characterization for geo-exchange purposes. Kavanaugh and Rafferty (1997) described site characterization steps for obtaining design parameters for various ground coupling options for commercial and institutional systems. Their work remains a useful, pragmatic reference document for pre-design studies (as well as geo-exchange design). Bridger and Allen (2005) described aquifer characterization methods for physical, hydraulic, thermal, and hydrochemical properties useful for aquifer thermal energy storage and included references to a number of other characterization and design studies. Canadian Standards Association (CSA) Standard C448 Series 02, Design and Installation of Earth Energy Systems (CSA 2002), describes site characterization steps and the scope for various field investigations (e.g., number of test boreholes per site, testing requirements, etc.). We view C448 as a minimum requirement for good practice; often there will be a need for additional effort driven by complex geology, large site size, or critical system design requirements (e.g., for integrated or hybrid geo-exchange systems). Significant contributions for specific field testing methods have been published (e.g., field tests of ground thermal properties [Kavanaugh 2000]). Sachs and Dinse (2000) present a site characterization decision flowchart along with descriptions of relationships of geology and ground heat exchangers (GHXs).
Site characterization studies address the question "could a site be used for geo-exchange purposes?" and identify site characteristics useful for design input parameters. The site suitability methods presented herein go further, addressing the question "should a site be considered for geo-exchange and, if so, which ground coupling is most appropriate?" This incorporates site characterization parameters as well as owner wishes, cost, schedule, and logistical, regulatory, and constructability factors into an owner-driven decision-making process.
Need for a Site Suitability Method
The main drivers for developing and using this method are:
* Selecting the best system for the site conditions.
* Delivering improved GHX value and reliability.
* Avoiding/reducing the risk of failed systems.
* Meeting growing expectations from owners for comparative GHX options.
* Meeting increasing regulatory demands.
* Meeting increasing certification demands.
Selecting an appropriate type of GHX for a project and site is perhaps the most important step in achieving a successful geo-exchange system (i.e., one that has an acceptable financial rate of return, provides efficient operation, meets regulatory requirements, and lasts a long time). Unfortunately, some past practice has focused on a pre-selected type of ground coupling (often by those with vested interests in the services and materials related to that type of GHX). This narrow outlook can lead to missed site opportunities for heat transfer or mediocre financial prospects, which may lead an owner to (wrongly) reject geo-exchange technology outright. A properly selected and sized GHX reduces the risk of inadequate ground coupling and system underperformance and unnecessary expenditures to install an overly large ground coupling as a conservative hedge against site uncertainty.
In geo-exchange, as in life, there is no free lunch. The understanding of a site (leading to good choices of GHX type and size) is directly related to the effort (time and money) invested in assessing site suitability. Also, site uncertainty, which can lead to overly conservative designs, is inversely proportional to the suitability assessment effort. Figure 1 depicts these relationships, which show the inherent value of obtaining appropriate site knowledge.
[FIGURE 1 OMITTED]
Detailed knowledge of every site is not always required if, for example, there are numerous other systems in an area and the local geology is uniform. However, where geologic and hydrogeologic conditions are complex, an appropriately thorough designer will want to know site conditions and comparative GHX options before proceeding to a design. Rules of thumb for approximating design parameters (e.g., length of vertical closed-loop borehole per ton or groundwater flow rate required per ton in open-loop systems) are useful when cautiously and prudently applied in early screening. However, most of these rules originated from residential applications in the midwestern and southern United States and can be misleading and risky when applied in areas of different geology, climate, and soil. This is especially true for commercial scale applications that can have distinctly different load profiles and ground thermal response characteristics than residential applications.
TYPES OF GHXs
GHX types considered in the GSA method are described in Table 1.
Table 1. Ground Heat Exchangers Considered in the GSA Method Type Description 1. Closed Loop Trench Ground coupling by an array of plastic pipe laid straight (single or multiple pipes) or in coils ("slinky" coils within trenches typically 1.5-2.5 m (5-8 ft) below grade; requires a relatively large site footprint compared with pre-heating ventilation makeup air). Shallow borehole Ground coupling by an array of shallow boreholes nominally up to 500 m (165 ft) deep with plastic pipe U-tubes grouted in, connected to header pipes; requires a moderate site footprint; subsurface investigations for this category would also pertain to energy piles (plastic pipe heat exchanger incorporated into building foundation piles). Deep borehole Ground coupling by an array of deep boreholds nominally >50 m (165 ft) deep with plastic pipe U-tubes grouted in, connected to header pipes; requires a smaller site footprint shallow borehole type. Surface water Ground coupling by a loosely bundled coil of plastic pipe or by a plate-style heat exchanger directly submerged in a body of water (ocean, lake, or river); typically with shallow buried header/transfer piping to the point of use; typically requires water bodies at least 4 m (16 ft) deep. 2. Open Loop Groundwater Ground coupling by pumping groundwater from a supply well though a plate heat exchanger (indirect system) or heat pump (direct system) then discharging to an injection well, infiltration pit, or surface point; site footprint typically smaller than closed-loop trench or borehole options. Surface water Ground coupling by pumping surface water from a lake or river to a plate heat exchanger (indirect system) or heat pump (directly system) then discharging back to stream or lake, injection well, infiltration pit, or surface point; requires proximity to surface water source; site footprint typically smaller than closed-loop trench or borehole options. Ocean water Ground coupling by pumping ocean water to a plate heat excharger (indirect system) then discharging back to ocean; requires a small site footprint and proximity to ocean source. 3 Waste Heat Sewage Conductive coupling to warm raw sewage stream by sewer pipes fitted with in-pipe or proximity heat exchangers (in soil backfill); typically in select urban settings with sustained sewage flows and nearby users. Process fluid Conductive coupling to process fluid (treated sewage effluent, industrial process water, landfill, leachate or mine dewatering discharge) by pipes fitted with in-pipe ro proximity heat exchangers (in soil backfill); typically in select industrial settings with sustained liquid flows and nearby users. 4 Hybrid Second source Some from of GHX combined with a second source of heating (solar collector orf boiler) or cooling (cooling tower); typically the GHX is smaller than would be needed for a stand-alone geo-exchange system and is used as a thermal "battery" as part of an integrated system (interchange of heat between multiple buildings or uses). Standing column Ground coupling by pumping groundwater from the (aka vertical bottom of a single well typically configured as a concentric well concentric well design to a heat exchanger or energy well) (indirect) or heat pump (direct) then discharging to the top of the same well; return water restored to ambient temperature during residence time in well; requires favorable groundwater chemistry; not common in British Columbia.
Factors Affecting Choice of GHX
It is valuable to optimize the type and size of a GHX for a given site for the following reasons:
1. Capital cost savings.
2. Operational cost savings.
3. Long-term thermal sustainability.
In many geo-exchange systems, the GHX is the single largest capital cost item. Depending on system size, the costs of different GHX options can vary by up to a factor of 10. For the most favorable financial payback, it is therefore wise to invest in determining site suitability, leading to the right choice and size of GHX. The risks of incorrectly sizing a GHX due to lack of site knowledge are depicted in Figure 2.
[FIGURE 2 OMITTED]
Importance of Geology for GHXs. The variability of geology affects the suitability of a geo-exchange system in two key ways:
* Thermal exchange properties vary widely for different geologic materials (soils, peat, rock), their degree of moisture content, and density; and
* The ease of drilling and GHX constructability can vary widely based on geology, material density, and ground-water occurrence which, in turn, greatly influences GHX installation cost.
Kavanaugh and Rafferty (1997) provide useful summaries of thermal properties for different geologic materials. Site-specific formation thermal conductivity testing may be included in intrusive site suitability assessments (see "Stage 2 GSA--Intrusive Work" later in this paper). Local geo-exchange, geotechnical, or water well drilling contractors are often the best source of information on ease of drilling in a particular area.
Importance of Hydrogeology for Ground Heat Exchangers. Hydrogeology is important for geo-exchange applications because 1) wet earth materials conduct heat significantly better than dry materials, 2) groundwater flow can carry heat from or to a site, and 3) abundant groundwater can be a valuable heat resource for open-loop systems (or for affecting thermal diffusivity around a closed-loop system) since the heat capacity of water is greater than that for solid earth materials. Soil moisture content is an important design parameter (for closed-loop systems) where groundwater is absent or very slow moving. Mineral precipitation (scaling), biofilm formation, and corrosion are commonly overlooked but are key reasons why water systems or heat exchangers can foul, underperform, or fail in open-loop groundwater GHX options.
Uncertainty in Earth Materials and Systems. Earth materials can be heterogeneous (showing spatially varying material properties) or anisotropic (showing directional material properties). Naturally occurring soil, rock, and groundwater are generally not uniform in their character, distribution, or behavior from site to site (or even within a site).
Area Availability. Available area for a GHX controls the selection of the GHX options that could sensibly fit on the site. Typically, large sites with abundant available area enjoy the widest range of potential GHX options. Depending on construction staging, areas beneath a building or structure may also be available for GHX installation. Therefore, the timing of when potential GHX areas become available is also important to know.
Site Access and Limitations. Site access, construction logistics, location of easements, legal limitations, and remoteness are all limiting factors in assessing site suitability. In addition, the proximity of a proposed GHX to established GHXs is important to know to assess possible interference effects.
Building Loads and Balance of Geo-Exchange Systems. GHXs are smallest and geo-exchange systems generally perform best for balanced ground loads (i.e., when the heat extracted from the earth is the same as the heat rejected to the earth on an average annual basis). It is important to know at the concept stage if ground loads will be balanced or to what degree they will be unbalanced. For this reason, peak and annual building block loads for both heating and cooling are included in the required owner information for assessing site suitability. In the absence of detailed building block loads, even simply knowing if the geo-exchange system is to provide only heating, only cooling, or both heating and cooling or is intended for some other use (e.g., domestic hot water) is useful. The professional can make reasonable assumptions as to the load balance from that information and the climate for the site area.
METHOD FOR ASSESSING SITE SUITABILITY AND GROUND COUPLING OPTIONS
General Approach
The approach presented here for assessing site suitability and ground coupling options is based on a three-stage GSA process, as depicted in Figure 3.
[FIGURE 3 OMITTED]
This rational, structured process proceeds from an owner's concept through progressive site suitability and pre-design stages with milestones involving owner decisions to the start of design. This process is based on providing the best value to the owner against a variable set of decision-making criteria with no preconceived selection of GHX type or configuration.
The GSA process relies on professional judgment as well as field observations, testing, analysis, and comparative evaluation, with adaptability and flexibility to be applied to simple, small projects and large, complex ones. Hallmarks of this process are that 1) a Stage 1 GSA,...
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