1,720,986 research outputs found
IMPROVING THE ASHRAE METHOD FOR VERTICAL GEOTHERMAL BOREFIELD DESIGN
No full textThe design of a system of Borehole Heat Exchangers (BHE) coupled to a geothermal heat pumps have to be done with a suitable dynamic model able to cope with the intrinsic time varying behaviour of the ground mass and building heat load profile. Among the models based on the solution of transient conduction equation, the Ashrae method (Kavanaugh and Rafferty) is probably the fastest algorithm for calculating the overall length of ground heat exchangers starting from the knowledge of the building thermal energy requirements and ground properties. This method employs Infinite Source (IS) solutions for describing the ground response to a series of three heat pulses, representing the building thermal history from the short to the long period. Since IS solutions cannot describe 2D and 3D effects in the ground temperature field, a correction parameter is introduced. This parameter is named Temperature Penalty Tp, which also accounts for the thermal interactions of neighbouring boreholes in the long term period.
In this paper a new method is proposed for the calculation of the Tp parameter. The new method is conceived for maintaining the simplicity of the original Ashrae model while enabling a more accurate design of the BHE field. The validation of the proposed procedure and the estimation of the constants related to the new method is made by assuming the Tp values as inferred from FLS generated g-functions, able to describe the ground response to a large number of BHE configurations, including square, rectangular, in-line, L-shaped, open rectangles. With reference to the present set of BHE configurations (about 120), it is demonstrated that the Ashrae Tp values are typically underestimating the “correct” value counterparts (average deviation more than 40%), thus leading to an underestimation of the BHE field overall extension. The proposed method, based on the calculation of a set of constants to be applied to specific geometries (square, rectangular, in line arrangements) is able to provide Tp values well centered around the benchmark line and with an average deviation of less than 8%, with estimated BHE overall lengths (ground heat extraction mode) very close (3%) to reference FLS values
Ground properties evaluation for the design of geothermal heat pump systems and uncertainty measurement during the Thermal Response Test
No full textThe exploitation of low enthalpy geothermal resources
for building heating and cooling purposes represents an important opportunity for saving energy in the domestic
and service sectors, that cover about 30% of the demand of
primary energy in the European Union. Vertical Borehole
heat exchangers (BHE) are the most frequently adopted solution
for ground coupled heat pump (GCHP) applications.
The heat transfer between the borefield and the surrounding
ground is driven by the ground thermophysical properties,
the borefield geometry and the temporal distribution of heating and cooling loads. For these reasons the estimation of the ground properties, thermal conductivity above all, is crucial in the borefield design analysis. The thermal response test (TRT) is well assessed technique for evaluating the ground conductivity together with the equivalent borehole resistance. In this paper the ground modeling and measurement analysis are discussed with respect to TRT experiments. Starting from a set of real TRT investigations carried out in different parts of Italy, an uncertainty analysis is presented with special attention devoted to the measurement errors and disturbances, including the important effects of the typical non steady condition of the applied heat transfer rate
FULLY ANALYTICAL FINITE LINE SOURCE SOLUTION FOR FAST CALCULATION OF TEMPERATURE RESPONSE FACTORS IN GEOTHERMAL HEAT PUMP BOREFIELD DESIGN
No full textGround-coupled heat pump (GCHP) systems very often employ vertical borehole heat exchangers (BHE) which are in charge of extracting or injecting heat from and to the ground by exploiting the conductive properties of the soil. The ground-to-BHE-field thermal interactions are a complex transient phenomenon that requires the knowledge of the building energy demand in time and a suitable engineering model for predicting the ground temperature variations in the short and long term. A computationally efficient way to tackle this problem is the recursive calculation of a basic thermal response factor for given different heat pulses representing the building energy demand. In this paper a review of the existing response factor models for BHE analysis is performed and the Finite Line Source (FLS) model is employed to develop and refine new fully analytical and explicit FLS solutions suitable for fast spatial and temporal superposition. The calculations of the temperature response functions are made for a great number of BHE configurations, including different layouts, from compact geometries (e.g. rectangular matrixes) to in-line and open arrangements. Comparisons are finally made among the reference and approximated solutions, also in terms of calculation time. This analysis shows that present analytical expressions can reduce the computation time of large borefield g-function generation down to 1% while maintaining an acceptable average error of about 3% with respect to literature analytical FLS solutions
Improved Ashrae method for BHE field design at 10 tear horizon
No full texttAmong the strategies for borehole heat exchanger (BHE) field design, the Ashrae method is a powerfuland fast procedure for the prediction of the BHE field length required by the building monthly heat loadprofile. The method needs the calculation of the Temperature Penalty, a key parameter that takes intoaccount the error introduced by the infinite cylindrical source (ICS) solution of the conduction problemwith respect to the “true” g-function solution. Recently the Authors proposed a modification of the originalAshrae (Kavanaugh and Rafferty) method which maintains the simplicity of the original Ashrae modelwhile dramatically improving the estimations of the required BHE field length and geometry withoutcalculating any g-function and with no need of special algorithms or computer codes. The new method(Tp8method) is centred on a scheme where eight heat sources are thermally interfering with the currentBHE source: optimum constants and new model validation had been calculated with reference to a setof more than 200 BHE design configurations described by the pertaining temperature response factors(i.e. g-functions). In this paper the validation of the Tp8model is further carried on in order to prove themodel robustness while the design input quantities (BHE depth, ground properties, mutual importance ofAshrae three load sequences) are changed to cover most of practical cases in borefield design. In particulara further simple term is introduced (and discussed) in Tp8Fourier number calculation which enables thepresent model to assure high accuracy also when the BHE depth is significantly different from usualvalues, say from short to very deep BHEs (60-240 meters)
Improving the Ashrae method for vertical geothermal borefield design
No full texttThe design of a borehole heat exchanger (BHE) field coupled to a geothermal heat pump requires a suit-able dynamic model able to take into account the intrinsic time varying behaviour of the building heatload profile and ground thermal response. In this paper a new method is proposed for the calculationof the Temperature Penalty, the key parameter of the Ashrae dynamic method originally developed byKavanaugh and Rafferty. The proposed method is conceived for maintaining the simplicity of the originalAshrae model while enabling a much more accurate design of the BHE field. The validation of the newprocedure is made with reference to a comprehensive set of 240 BHE configurations, described in termsof the corresponding temperature response factors (i.e. g-functions). It is demonstrated that the Ashraetemperature penalty values typically underestimate the corresponding true values with an average devi-ation of more than 40%. On the other hand the proposed method is able to provide temperature penaltyvalues well centered around the benchmark line and with an average deviation of less than 10%; it isfinally demonstrated that the present procedure is also much more accurate than other existing modelsand simpler to apply in engineering design
An improved method for vertical geothermal borefield design using the Temperature Penalty approach
No full textThe Ashrae method (Kavanaugh and Rafferty) is one
of the few engineering models that allows a system
Borehole Heat Exchangers (BHE) to be quickly
designed starting from the knowledge of the building
thermal energy requirements. The method is based on
Infinite Source solutions from ground dynamic
response to a series of three heat pulses, representing
the building thermal history from the short to the long
period. The key parameter of the Ashrae procedure
(recently adopted also as an Italian Standard) is the
evaluation of the Temperature Penalty correction Tp,
which takes into account the thermal interactions of
neighbour boreholes in the long term period.
In this paper a new method is addressed to the
calculation of the Tp parameter and it refers to a
physically based approach of mutual interactions
among the BHEs. The improved method has been
conceived for maintaining the simplicity of the
original Ashrae scheme while enabling a more
accurate estimation of the Temperature Penalty values
and hence a more reliable BHE field design data. The
validation of the proposed procedure and the
estimation of the constants involved in the new Tp8
method is based on the “exact” calculation of the Tp
values starting from FLS generated g-functions, able
to describe the ground response of a large number of
BHE configurations, including square, rectangular, inline,
L-shaped, open rectangles.
It is demonstrated that for the set BHE configurations
here considered (about 120) the average deviation of
the Ashrae Tp values (with respect to the FLS
benchmark) is above 46% with a typical
underestimating behaviour which reflects in
underestimating the BHE field overall length. On the
other hand the proposed method yields Tp percentage
deviations well centered around the benchmark line
and with an average deviation of 18%. The new
method, with respect to benchmark set and in heating
mode operations, is able to yield the design BHE
length within 5% with respect to the reference
solutions
A Web Application for Geothermal Borefield Design
No full textThe correct design of a borehole heat exchanger (BHE) system implies the accurate knowledge
properties, the correct evaluation of building heating or cooling demands and the correct
length related to BHE configuration shape. A careful design is required to make profitable
time performance. In this paper the principle and description of a web-based suite of tools for BHE design is
implement some of the main procedures universally adopted in BHE system design. A
implementation of an improved ASHRAE method which allows the BHE system to be designed
building heat loads, their respective thermal resistances based on ICS model and a temperature
account the long term BHE interaction effect. A new hybrid implementation of the Ashrae
is also described for arbitrary borehole field configuration design. This arbitrary shaped
the peculiar needs of each application project. Current free or commercial programs
according to a priori determined set of available configurations in terms of borefield shape
real applications often require the borehole field geometry not to respect a regular matrix
a toolkit for Thermal Response Test (TRT) analysis which allows the estimation of BHE
toolkit provides also a method for the evaluation of the error related to TRT analysis results.
estimation of borehole thermal resistance based on literature review is also presented
EXPERIMENTS WITH COAXIAL FITTINGS FOR ENHANCING THE PHASE DISTRIBUTION IN PARALLEL VERTICAL CHANNELS
No full textIn this paper, in order to establish the influence of the operating conditions and of the header geometry on the phase/mass distribution into parallel vertical channels, a number of flow distribution fittings have been tested with respect to a test section characterized by a horizontal header and 16 vertical upward vertical channels. The fluids are air and water and the flow regimes at the header cover the intermittent and annular flow pattern conditions. The effects of the operating conditions, the header geometry were investigated in the ranges of liquid and gas superficial velocities of 0.2-1.2 and 1.5-12 m/s, respectively. The header geometries here compared are the smooth reference case, with a series of cylindrical distributors with in-line holes along its length (“flutes”). New data in particular are presented with respect to the “double chamber flute” fitting. As in previous studies by the present research group, extracted mass flow rates of liquid and gas from parallel channels are recast in terms of normalised flow rates and normalised standard deviation of phases
Numerical evaluation of the Ground Response to a Thermal Response Test experiment
No full textA prerequisite for the correct design of vertical ground
heat exchangers (or Borehole Heat Exchangers, BHE)
for heat pump applications is the knowledge of the
ground thermal properties, in particular the thermal
conductivity.
The Thermal Response Test is a well known experimental procedure that allows the ground thermal and the BHE thermal resistance to be evaluated. A TRT is performed by providing a known and constant thermal power to a fluid (usually water) that circulates through a BHE buried in the site of interest; the water
temperature measurements, which varies over time, represent the data to be analyzed in order to solve an inverse conduction problem. The standard analysis method addressed to parameter estimation is based on the Infinite Line Source (ILS) model.
In the present paper different 3D numerical models,
are developed in order to numerically describe a TRT
experiment. The calculation environment is Comsol
Multiphysics® and either the thermal conduction
inside the ground and grout or the fluid to pipes
interactions are taken into account. The results of the
simulations have been employed for a back evaluation
of the ground thermal conductivity according to the
standard ILS approach, to infer useful information on
the errors in parameter calculation and to check the
estimation capabilities of a new method based on
temporal superposition and optimum search. The
proposed method is in particular able to cope with
situations where the TRT experiments are related to
highly variable heat transfer rates to the carrier fluid.
The present results show that the proposed approach is
very reliable alternative to the standard ILS approach
and that BHE parameters can be estim
percent error with respect to reference values
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