Nikolaos Taousanidis 1* and Elisavet Amanatidou 2

Transcription

1 Nikolaos Taousanidis 1* and Elisavet Amanatidou 2 1 Department of Mechanical Engineering and Industrial Design, Technological Education Institution of Western Macedonia, Kozani 50100, Greece, address: taousan@teiwm.gr 2 Department of Environmental Engineering and Pollution Control, Technological Education Institution of Western Macedonia, Kozani 50100, Greece Abstract: Solar combisystems are investigated in this paper for possible use in SE Europe countries. The study is based on a simulation study of a model developed according the existing solar applications in these countries. The analysis indicates that there is an interesting potential for applications and correlates this with collector area which is the crucial parameter of the system. Finally an economic analysis is presented which justifies the cost invested for such an application. Keywords: solar combisystems, simulation model, fractional solar consumption, life cycle cost Solar heating systems which provide both domestic hot water and space heating, so called solar combisystems or SDHW&H systems, are increasing their market share in several countries such as Austria, Germany, Denmark, the Netherlands and Switzerland. In some countries, such as Sweden, they have been the dominant solar system type for a long time. In a combisystem there are at least two energy sources used to supply heat to the two heat consumers: the solar collectors deliver heat as long as solar is available, and the auxiliary energy source (oil, gas, wood, electricity, etc.) supplements the missing solar. As a general rule, collectors should be operated at the lowest possible temperature in order to have a good efficiency; at high temperature they have significant heat losses. Other individual requirements arise from the auxiliary heat source selected.

2 86 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU Much is already known about solar domestic hot water systems, but solar combisystems are more complex and have interactions with extra subsystems. These interactions profoundly affect the overall performance of the solar part of the system. The general complexity of solar combisystems has led to the development of a large number of widely differing system designs, many only very recently introduced onto the market. After the first period of combisystems ( ), where design of non-standard and complex systems by engineers was the rule, a new period has been opened since Now essentially solar companies trying to sell simpler and cheaper systems do the design. But current designs are based mainly on field experiences and they have not yet been carefully optimised. Experts believe that there is a great potential for cost reduction, performance improvement and increase in reliability, and that this needs to be scientifically addressed. The combination of thermally well-insulated buildings, lowtemperature heat emission systems and solar heating systems with short-term heat storage, allows the heating requirements of a singleor multi-family dwelling to be met in a user-friendly way at an acceptable cost. In comparison to systems with a seasonal storage, the costs of which are currently not justifiable for single-family houses, this combination is more cost-effective. The demand for solar combisystems is increasing rapidly in several countries. However these countries are mainly central and northern Europe countries. There is a complete absence not only of a serious effort for a commercial application but also for a detailed research work in this field in southeastern Europe. This paper deals with the applicability of solar combisystems in Balkan Peninsula countries (specifically in Greece and Romania) which enjoy an abundance of solar energy and justified use of these systems. Considerations for design practices and system optimization are encountered within. There are a number of generic designs for solar combisystems proposed in the largest research work, done in this field, by IEA- SHC Task 26 (Suter et al., 2000). The system which is proposed for Balkan countries is one closer to #2, because it doesn t need a lot of

3 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 87 changes from the traditional systems which exist in these countries. In the existing solar system with the bivalent heat store (or trivalent if electric energy is used) we add an external heat exchanger used for solar preheating of the return flow from the conventional space heating system and the system looks like that in Figure 1. Figure 1: Proposed Solar Combisystem for South-eastern European Countries The main barrier for large scale introduction of solar thermal systems is the high capital cost compared to conventional heating systems. Especially for the solar combisystems which for singlefamily houses have in practice collector areas between 8 and 30m 2 and storage sizes of l compared to solar domestic hot water (DHW) systems with 4 8m 2 and l, respectively the cost is considerably higher and is one of the main parameters governing their design. The configuration in figure 1 is that we believe gives the lower cost as it requires minor changes from the common solar DHW systems. Practically the useful solar energy is used for DHW storage tank heating (mode 2) until the tank base temperature T tank,b reaches a certain temperature. If this requirement is satisfied and there is a space heating demand, solar collector preheats the return flow from space heating delivery system (usually radiators in these countries) through an external (usually plate) heat exchanger (mode 1). The auxiliary heating is provided with an oil boiler either in order to reach the tank top temperature T tank,t at a certain level (mode

4 88 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU 2), or (if this is satisfied and there is need for space heating) to cover space heating load. The above described operation is maintained through two controllers which according the values of collector output temperature, return flow temperature and the two mentioned tank temperatures control the position of two 3-way valves and the operation of two circulating pumps (Taoussanidis and Gavros, 2003). All main energy flows go through the heat storage in mode 2 operation. Even mode 1 operation can be incorporated and described by the following general formula: where Q u Q aux E = Q u + Q aux Q load - Q loss (1) collector s useful energy boiler s auxiliary energy Q load DHW and space heating load (Q DHW + Q SH ) Q loss E storage tank losses change in tank s internal energy The collector s useful energy is given by the well known Hottel- Whillier-Bliss equation Q u = A c [F R ( * ) n I T - F R U L (T o -T a )] (2) where A c F R I T U L T o T a collector s area collector heat removal factor transparent cover s transmissivity absorbing surface absorptance radiation flux on the collector plane collector heat loss factor collector s output temperature ambient temperature

5 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 89 where The auxiliary energy can be derived by Q aux = m oil H u n boiler (3) m oil mass flow of consumed oil H- u lower calorific value of oil n boiler boiler s efficiency The domestic hot water load is given by Q DHW = m DHW c p (T c -T h ) (4) where m DHW c p T c T h where mass flow of DHW specific heat of water cold water temperature hot water temperature The space heating load can be derived by Q SH = m SH c p (T s -T r ) (5) m SH c p T s T r where mass flow in space heating distribution system specific heat of water supply temperature return temperature The storage tank losses can be approximated by Q losses = (UA) s.dhw * (T st,dhw - T loc ) (6) (UA) S,DHW = 0,16 V S, DHW : heat loss rate (W/K) (according prenv :2000). (7) V s,dhw : the storage volume T st,dhw : storage temperature T loc : room temperature where the tank is located Most of the variables above are time dependent (temperatures, meteorological data and so on). Hence a thorough evaluation of a

6 90 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU solar combisystem needs to be done on a time basis (Hasan, 1999), (Henden et al., 2002), (Lund, 2005). A yearly performance considered to be a satisfactory judgment criterion. We use the TRNSYS 15 (Klein et al, 2000) simulation program. The meteorological data are provided for the two regions (Kozani (GR) and Agigea (RO)) for typical years on hourly basis (Figures 1 and 2). The insolation model we use developed by Reindl et al. (1990). It estimates the fraction of diffuse to total horizontal radiation according the equation Figure 1: Ambient Temperature (C) Distribution throughout the Year for Kozani (Grey) and Agigea (Black) Id I or Id I or Id I k sin k sin 0.486k sin T T T for 0 k T 0,3 and I d /I 1 (8a) for 0.3 k T 0.78 and 0.1 I d / I 0.97 (8b) for 0.78 k T and 0.1 I d /I (8c)

7 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 91 where k T is the ratio of total radiation on a horizontal surface to extraterrestrial radiation (clearness index) Figure 2: Solar Radiation on Horizontal (W/m 2 ) Distribution throughout the Year for Kozani (Grey) and Agigea (Black) Figure 3: Space Heating Load (kj/hr) Distribution throughout the Year for Kozani (Grey) and Agigea (Black)

8 92 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU As far as the contribution of diffuse radiation is considered we use the isotropic sky model (Hottel and Woertz, 1942). The building we use to estimate space heating loads is a twostorey office building with a total area of 180 m 2. The space heating load is calculated with Prebid (building input description program for TRNSYS) for the two climates as shown in figure 3. The DHW load is around 160 l/h and follows the ANSI/ ASHRAE Standard (1993) consumption distribution The solar combisystem, which will be used as a basis for the verification of the model, consists of four solar collectors connected in series with a total area of m 2 and a storage tank of 420 litres. As the main point of this article is to justify the use (or not) of a solar combisystem in Balkan countries we will use Fractional Solar Consumption (FSC) characterisation procedure which developed under the frame of IEA-SHC Task 26 (Letz, 2002). In that way we can compare results which one would obtain by placing the same combisystem in different climates and other buildings. Furthermore, the characterization allows comparing different solar combisystems. Fractional Solar Consumption (FSC) represents the proportion between fossil energy consumption that a solar system could be theoretically saved and the total fossil energy consumption of a conventional reference installation FSC is a dimensionless quantity simultaneously taking into account the climate, the building (space heating and domestic hot water loads) and the size of the collector area, in a way that doesn t depend on the studied SCS. FSC is calculated on a monthly basis in a simple way, using the solar collector area A c (m 2 ), the monthly global irradiation in the collector plane H (kwh/m 2 ) and the monthly reference consumption without solar combisystem Q load (kwh) : FSC 12 1 min( Q, A H) 12 1 Q load load C (9)

9 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 93 In the model we built, we calculated FSC for the two climates and for different collector areas which is the main parameter which affects FSC but also the cost of the system. The results are shown in Figure 4. Figure 4: Fractional Solar Consumption Sensitivity Analysis Results for Kozani (Grey) and Agigea (Black) A thorough elaboration of these results has shown that there is a logarithmic relationship which is For Kozani: FSC = ln A c with R 2 = (10a) For Agigea: FSC = ln A c with R 2 = (10b) The R 2 values show the very good approximation of the functions to the results and the subsequent usability of the formulas. The values of FSC indicate the vast potential of SCS for the two countries and the applicability of these systems at least on energy basis. In next paragraph we try to prove that there is also an economic basis of this justification.

10 94 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU We will analyse the existing system for one of the cases (Kozani) taking into account economic figures which are common in both countries. The objective of the economic analysis can be viewed as the determination of the least cost method of meeting the energy need, considering solar and auxiliary energy. The problem is to determine the lowest cost combination of solar and auxiliary energy. In the proposed combisystem, the main impact of solar energy is the increased mortgage payments and decreased fuel costs. Solar savings are the difference between the cost of conventional energy and solar combisystem and is given by savings = (costs of conventional energy) (costs of solar energy) For the economic analysis in the proposed combined system, the life cycle savings (LCS) method is used (Duffie and Beckmann, 1991). This method takes into account the time value of money and allows detailed consideration of the complete range of costs. Fuel expense is for energy purchase. The mortgage payment includes interest and principal payment on funds borrowed to install the system. Maintenance and insurance are recurring costs required to keep the system in operating condition and keep it protected against fire or other losses. Parasitic energy costs are electrical and mechanical energy uses in the system. Income tax savings for a nonincome producing system can be expressed as (income tax savings) = (effective tax rate) X (interest payment + property tax) If an obligation recurs each year and inflates at a rate of i per period, present worth factor (PWF) of the series of N such payments can be found from the following equation: j i N (1 i) [1 ] if i d PWF( N, i, d) j ( d i) 1 d j 1 (1 d) N /(1 i) if i d N (11) where N is the period of economic analysis, i is inflation rate and d is the discount rate.

11 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 95 The relationship for determining the present worth of 1 Euro needed N periods (usually years) in the future, with a market discount rate of d, and is given by PW 1 (1 d) N (12) The solar combisystem is estimated to provide annual thermal energy of 13,000 kwh (th) per year or 21% of the total annual energy demand (61,700 kwh). The solar part of the combisystem (space heating system exists) costs 4160 Euros and the total system Euros. Suppose that it is 90% financed over 15 years at an interest rate of 9%. The cost of conventional energy is estimated at about 0.05 Euro/kWh after taking into consideration parameters referring to installation, operational and investment costs. The first year s oil cost (without solar system energy) is estimated at approximately 3085 Euro. According to the market s pricing policy, the cost of the oil is expected to rise at 10% per year. Furthermore, it is expected that the equipment will have a resale value at the end of 15 years of 40% of the original cost. In the first year of operation, extra insurance, maintenance and parasitic energy costs are estimated as 1% of the total investment cost. This cost is expected to rise at a general inflation rate of 5% per year. The effective income tax rate is expected to be 40% through the period of analysis. If the market discount rate is 8%, then the present worth of solar savings for this application over a 15-year period is given in Table 1. The resale value of 4400 in year 15 is shown as the second entry in entry 15 and is positive as it contributes to savings. The sum of last column (2625 Euros) is the total present worth of the gains from the solar combisystem compared to the conventional system. From the economic analysis of this system, it was found that the year-to-positive cash flow is 5 years. I t has to be noted that we have accepted annual savings of 21% which is a moderate estim ate, as FSC characterisation procedure the theoretical potential of savings is at least double.

12 96 / NIKOLAOS TAOUSANIDIS AND ELISAVET AMANATIDOU Table 1 Economic Analysis of the Combisystem in Euros Income Solar PWof Fuel ExtraMort. Maint., Tax Savings Solar Year Savings Pt. Energy Savings Saving ,4-323, ,4-275, ,2-222, ,8-164, Total present worth of solar savings 2625 A solar combisystem configuration, ideal for Balkan countries has been investigated in order to examine its applicability. First, a simple energy balance analysis was outlined and a simulation model derived accordingly to study the effects of different sizing parameters and driving factors of solar combisystems. The equations for Fractional Solar Consumption estimation with high proximity (R 2 >99%) have been presented. It has been proved that there is a ground for such application both from the energy and economic analysis. The study indicates that with present and near term cost structure of solar heating systems their installation and operation can be economically justified. Comparing the calculated solar combisystem performance values in Greece and Romania showed that there aren t noticeable differences and the outcomes are valid for both. However the model has to be validated and specific cases, as other components effects on the total system performance, have to

13 A STUDY ON THE DESIGN AND ANALYSIS OF A SOLAR PRODUCT... / 97 be studied. Thus, a comprehensive sensitivity analysis of the solar combisystem sizing against a multitude of parameters included in the numerical simulations is necessary. Another aspect to be studied is if in low energy houses, going for solar combisystems would require system cost which may be possible on a longer term. [1] ANSI/ASHRAE Standard Energy Efficient Design of Low-Rise Residential Buildings, Section 8.9.4, Hourly Domestic Hot Water Load Profile and Table 8-4, Daily Domestic Hot Water Profile pp American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta,GA, USA [2] Duffie J.A. and Beckman W.A., Solar Engineering of Thermal Processes, ed. John Wiley & Sons, [3] Hasan A., Sizing Solar Space Heating System: A Case Study, Renewable Energy 16, pp , [4] Henden L. Rekstad J. and Meir M., Thermal Performance of Combined Solar Systems with Different Collector Efficiencies, Solar Energy, vol 72. No 4, pp , [5] Hottel H.C. and Woertz B.B., Performance of Flat-Plate solar Heat Collectors, Trans ASME, 64, 91, [6] Klein S. A. et al., TRNSYS A Transient System Simulation Programversion 15.00, Solar Energy Laboratory, university of Wisconsin-Madison, USA, [7] Letz T., Validation and background information on the FSC procedure, Report of IEA SHC Task 26 Solar Combisystems, December [8] Lund P.D., Sizing and Applicability Considerations of Solar Combisystems, Solar Energy, vol. 78, No. 1, pp , [9] Reindl D.T., Beckman W.A. and Duffie J.A, Diffuse Fraction Correlations, Solar Energy, vol. 45. No 1, [10] Suter J-M, Letz Th, Weiss W. and Inaebnit J, Solar Combisystems in Austria, Denmark, Finland, France, Germany, Sweden, Switzerland, the Netherlands and USA-Overview 2000, IEA SHC-Task 26 Solar Combisystems, [11] Taoussanidis N., Gavros C., Modelling, Sensitivity and Optimisation of a Solar Combisystem METSIM 2003 International Conference on Metrology & Measurement Systems, 2003.