AN EFFECT OF FLOW NON-UNIFORMITY IN EARTH-TO-AIR MULTI-PIPE HEAT EXCHANGERS (EAHEs) ON THEIR THERMAL PERFORMANCE

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1 Łukasz AMANOWICZ *, Janusz WOJTKOWIAK * Ground earth-to-air multi-pipe heat exchanger, flow rate non-uniformity, thermal performance AN EFFECT OF FLOW NON-UNIFORMITY IN EARTH-TO-AIR MULTI-PIPE HEAT EXCHANGERS (EAHEs) ON THEIR THERMAL PERFORMANCE Recent flow characteristic investigations of multi-pipe earth-to-air heat exchangers (EAHEs) revealed non-uniformity of flow rate in each branch of exchangers having traditional structure. In this paper an effect of the flow rate non-uniformity on a thermal performance of EAHEs is investigated theoretically using simple and accurate method for engineering application. Results show, that the total thermal performance of real ground heat exchanger having traditional structure is about 10% lower than for theoretical exchanger. Additional calculations for real exchanger having modified (improved) structure show that a simple and free of cost change in exchanger structure (resulting in a better flow uniformity) provide about 9% more heat than real traditional structure. 1. INTRODUCTION 1.1. EARTH-TO-AIR MULTI-PIPE HEAT EXCHANGERS Reduction of energy demand in new buildings caused a necessity of energy recuperation from removing air. This solution is commonly used in ventilation systems, not only in detached houses, but also in office and industrial buildings. Ventilation units with plate heat exchangers achieve thermal efficiency at about 70-80%, but operate stable if outside temperature is higher than about -4 C [6]. When outside temperature decreases, plates freeze up, what makes system unstable and not so energy efficient as it could be. To avoid that situation and to gain heat (or cool) from the ground, EAHEs are used. For high demand of fresh air (office buildings, shopping centers) multi-pipe exchangers are preferred to diminish total pressure losses and to make the system more energy efficient. * Instytut Inżynierii Środowiska, Politechnika Poznańska, ul. Piotrowo 3a, Poznań

2 1.2. AIM OF PAPER Usually, whole year operation analysis of multi-pipe EAHEs is made with assumption of uniform division of air between all parallel pipes. Recent experimental investigations of EAHEs models revealed non-uniformity of flow rate in branches of exchangers having traditional structure. The aim of this paper is to calculate how does the non-uniformity influences on a thermal efficiency of EAHEs designed with assumption of uniform air division. A simple and accurate model, taking into account a change of ground temperature during a year, a kind of a ground and hourly changing climate data is used to calculate the total amount of heat gained by means of EAHE during a typical year. Calculations are done for 3 structures of multi-pipe exchangers: traditional theoretical structure with assumption of flow rate uniformity, real structure with experimentally verified flow rate non-uniformity and modified structure (also investigated experimentally). The last structure differ from traditional structure in different air inlet location (Fig. 1). That simple change in an exchanger structure results with better flow rate uniformity [1] and lower pressure losses [2] FLOW RATE NON-UNIFORMITY In papers [1, 2] results of experimental investigations of EAHEs flow characteristics are presented. Measurements of flow rate and pressure losses in each branch of multi-pipe EAHEs were carried out. EAHEs of two different structures (Fig. 1) and various number of parallel pipes were investigated. Results presented in papers [1, 2] show that modified parallel structure is better than the traditional structure both from the branches flow rate uniformity as well as from the pressure losses point of view. Figure 2 shows examples of a percentage share of air flow rate in each parallel pipe in a total flow rate for traditional (S1) and modified structure (S2). In the modified structure flow rate uniformity is significantly better. Various flow rates implicate different Nusselt numbers and different heat transfer coefficients at the inner surfaces of parallel pipes. As a result thermal efficiency of EAHE depends on an air flow uniformity. S1 S2 Rys. 1. Układ tradycyjny (S1) i układ zmodyfikowany (S2) Fig. 1. Traditional structure (S1) and modified structure (S2) of EAHE

3 52% 15% 33% 37% 33% 30% Rys. 2. Procentowy udział przepływu w danej gałęzi V i w przepływie całkowitym V tot. 3 i 5 równoległych gałęzi: układ tradycyjny (S1) i układ zmodyfikowany (S2), [1] Fig. 2. Percentage share of each branch air flow V i in total air flow V tot. 3 and 5 parallel pipes: traditional structure (S1) and modified (improved) structure (S2), [1] 5; 50% 5% 4% 4; 30% 11% S1 S2 S1 S2 5; 30% 4; 25% 12% 21% 12% 2. MODEL AND ASSUMPTIONS Nomenclature: a g ground thermal diffusivity, m 2 /s c p air specific heat, J/(kgK) d pipe internal diameter, m H depth of EAHE location, m L length of single branch pipe, m air mass flow rate, kg/s Q energy, kwh R i thermal resistance of: i=w(pipe surface), i=d (pipe wall), i=g (ground) t N outlet air temperature, C t Z inlet air temperature, C U total heat transfer coefficient, W/(mK) λ g ground thermal conductivity, W/(mK) ρ g ground density, kg/m 3 Nu Nusselt number, - Pr Prandtl number, - A simple and accurate model for whole year analysis of EAHE operation is used. The most important formulas of the model were taken from [7]. Some of them are presented below. Air temperature at the outlet of EAHE: Rys. 3. Schemat przyjętego modelu: IN1 czerpnia terenowa, IN2 czerpnia., VU centrala went. Fig. 3. Schema of assumed model: IN1 air terrain inlet, IN2 air wall inlet, VU ventilation unit exp (1)

4 Total heat transfer coefficient: (2) Ground thermal resistance R g was calculated using formulas presented in [3]. Nusselt number at the inner pipe surface: 0,024,, 1+, ( / ), (3) Main assumptions: air is dry (steam condensation is not taken into account), thermal properties of air are calculated at (tz + tg)/2 using formulas taken from [4], natural undisturbed temperature distribution in ground is calculated using modified Baggs formula [5], ground thermal parameters for dry sand and for wet sand are taken from [8], EAHE operates during a day with various flow rates, in hours 22-6: 70%, 7-14: 50%, 15-21: 100% of maximum flow rate, EAHE operates if outside air temperature is lower than ground temperature at least of 2,0K (winter) or if outside air temperature is higher than 24 C (summer). Tabela 1. Właściwości cieplne rozważanych gruntów, [8] Table 1. Thermal parameters of considered grounds, [8] Ground ρ g [kg/m 3 ] λ g [W/(mK)] a g [m 2 /s] c g [J/(kgK)] humidity 10% ,77 11,2E humidity 30% ,70 11,2E RESULTS Calculations of heat gained by exchanger with 5 parallel pipes (DN200, L=15 or 30 m) were performed for 3 cases: A) theoretical traditional structure (total air flow divided uniformly into all branches), B) real traditional structure (air flow in each branch was taken from [1]), C) modified structure ( air flow in each branch was taken from [1]). Results of calculations are presented in table 2 and in Fig. 4. The differences between energy gains are calculated as follows: Δ = 100%, Δ = 100%, Δ = 100% (4) A-B A-C C-B difference between the theoretical and the real traditional structures difference between the theoretical traditional and the real modified structures difference between the real modified and the real traditional structures

5 Tabela 2. Procentowe różnice między ilością ciepła uzyskaną dla rozpatrywanych układów Table 2. Percentage differences between heat gained in considered cases Case Ground λ g [W/(mK)] L [m] H [m] A-B [%] A-C [%] C-B [%] 1 1, ,77 1, humidity 10% 3 2, , ,70 1, humidity 30% 6 2, Average: , ,77 1, humidity 10% 9 2, , ,70 1, humidity 30% 12 2, Average: Q [kwh] Case traditional structure (uniform) traditional structure (non-uniform) modified structure (non-uniform) Rys. 4. Ciepło, uzyskane w ciągu roku za pomocą gruntowego wymiennika ciepła zbudowanego z 5 równoległych gałęzi, przepływ 450 m 3 /h Fig. 4. Heat gained during a year by exchanger having 5 parallel branches, air flow 450 m 3 /h 4. CONCLUSIONS In spite of the limited scope of the study, the following conclusions can be drown: flow uniformity has noticeable effect on the total heat gained by EAHE, differences in heat obtained with assumption of perfect flow uniformity and for experimentally obtained non-uniformity in considered cases vary from 7 to13%, the above differences are higher for ground with lower thermal conductivity, modified structure provide about 9% more heat than real traditional structure.

6 LITERATURE [1] Amanowicz Ł, Wojtkowiak J., Badania eksperymentalne wpływu zmian sposobu zasilania powietrznego gruntowego wymiennika ciepła typu rurowego na jego charakterystykę przepływową. Cz. 1. Równomierność rozpływu, COW 6/2010, p , 220 [2] Amanowicz Ł, Wojtkowiak J., Badania eksperymentalne wpływu zmian sposobu zasilania powietrznego gruntowego wymiennika ciepła typu rurowego na jego charakterystykę przepływową. Cz. 2. Straty ciśnienia, COW 7-8/2010, p , 282 [3] Bose J.E., Parker J.D., McQuiston F.C., Design/data manual for closed loop groundcoupled heat pump systems, ASHRAE. Atlanta 1985 [4] Popiel C.O., Wojtkowiak J., Eksperymenty w wymianie ciepła, Wydawnictwo Politechniki Poznańskiej, Poznań 2004 [5] Popiel C.O., Wojtkowiak J, Biernacka B., Measurements of temperature distribution in ground, Experimental Thermal and Fluid Science Volume 25, Issue 5, 2001, p [6] Rosiński M., Odzyskiwanie ciepła w wybranych technologiach inżynierii środowiska, OWPW, Warszawa 2008 [7] Szymański M., Wojtkowiak J., Uproszczona metoda wymiarowania i oceny opłacalności gruntowego wymiennika ciepła w układzie wentylacji budynku, COW 7-8/2007, p.55-58,63 [8] Usowicz B., Szacowanie cieplnych właściwości gleby, Acta Agrophysica, Vol. 72, 2002, p STRESZCZENIE Wpływ nierówności przepływu w gałęziach na efektywność cieplną powietrznych wielorurowych gruntowych wymienników ciepła Badania doświadczalne powietrznych wielorurowych gruntowych wymienników ciepła o strukturze tradycyjnej, wykonane w Instytucie Inżynierii Środowiska Politechniki Poznańskiej, ujawniły dużą nierównomierność strumieni powietrza w gałęziach wymiennika. W referacie przedstawiono wyniki obliczeń wpływu stwierdzonej nierównomierności na efektywność cieplną wymienników o strukturze tradycyjnej i zmodyfikowanej. Obliczenia rocznej energii pozyskiwanej z gruntu wykonano dla różnych rodzajów gruntu, zmiennej głębokości posadowienia wymienników oraz różnych długości równoległych gałęzi. Stwierdzono, że założenie o równości strumieni powietrza w poszczególnych gałęziach skutkuje ok. 10% zawyżeniem efektywności cieplnej wymiennika o strukturze tradycyjnej w porównaniu z efektywnością wyliczaną dla rzeczywistego (doświadczalnie stwierdzonego) rozkładu strumieni na poszczególne gałęzie. Dodatkowo ustalono, że prosta modyfikacja sposobu doprowadzenia powietrza do wymiennika, zwiększa równomierność strumieni powietrza i skutkuje wzrostem jego efektywności cieplnej o blisko 9%.