SOLAR COMBISYSTEMS IN DENMARK SOLAR & BIOMASS SYSTEMS

Transcription

1 SOLAR COMBISYSTEMS IN DENMARK SOLAR & BIOMASS SYSTEMS Line L. Overgaard Solar Energy Center Denmark, Danish Technological Institute, Teknologiparken, DK-8 Århus, Denmark, , , Klaus Ellehauge Solar Energy Center Denmark, Danish Technological Institute, Teknologiparken, DK-8 Århus, Denmark, , , Abstract A project has been carried out to investigate and monitor different types of small Solar & Biomass systems for single-family houses in Denmark. The purpose of the work has been to obtain greater knowledge about the behavior of Solar & Biomass systems using common storage tank(s), to get an overview of system designs and to prepare guidelines for good system design. Inspections of 12 systems have revealed both poor system designs with basic faults like under- and oversized components and insufficient insulation, as well as well-designed and well-functioning systems. Monitoring of three of the systems over a period of almost a year have shown great differences with regard to a lot of different factors such as operation conditions, heat losses from installations, solar contributions and number of days the biomass boiler can be turned off each year. 1. INTRODUCTION Within recent years different system designs combining Solar and Biomass heating have been seen in Denmark. The use of common storage tank(s) should make it possible to reduce both investment costs and number of days the biomass boiler is used each year. The latter because the systems are designed to supply both space heating and domestic hot water demands. However, it requires that the system is designed and controlled properly, so that the two energy sources are not in each other s way. A project, funded by The Danish Energy Agency, has been carried out to investigate and monitor different types of small Solar & Biomass systems for single-family houses in Denmark. Prior to this project there have been no documented experiences with the Danish small-scale system designs combining solar heating and biomassfired boilers. A total of 12 systems have been inspected and thorough measurements have been carried out on of the systems. This paper gives an idea of different Danish system designs and presents results of inspections and measurements carried out in the project. An evaluation of different systems designs and guidelines for good system design are also given. 2. DANISH SYSTEM DESIGNS The 12 investigated Danish systems have been selected so they represent both technically well-designed systems, systems with components that have a great market share, as well as ill-considered system designs. The systems also represent different sizes and types of small Solar & Biomass systems, with different types of storage designs, different ways of preparing domestic hot water (DHW), different types of auxiliary energy sources, and different types of biomass boilers. Collector areas vary from 6-16 m², total storage volumes vary from.-2. m³, and nominal outputs of the biomass boilers vary from 2- kw. Most systems supply singlefamily houses with 2-4 occupants. One system supplies a multifamily house with 6-1 occupants. Table 1 gives an overview of the inspected systems. The different storage types, types of auxiliary energy source, and boiler types are described and commented on in section Storage designs and DHW preparation Three different types of storage designs occur in the 12 inspected systems: 1. 1 storage tank, for solar heating only 2. 1 storage tank, common. 2 storage tanks, connected Systems with one storage tank for solar heating only act as reference systems. All the systems with two storage tanks have the tanks connected in parallel. Most of the storage designs include tank-in-tanks with a submerged tank (sub) for DHW preparation. There are two different sizes of tank-in-tank storage on the Danish market with total storage volumes of. and.7 m³, respectively. They are, however, both too small to contain the surplus energy from the biomass boiler, and an extra storage tank is therefore often added to the system. Outside district heating areas there are given subsidies to biomass boilers and solar heating systems. Combined Solar & Biomass systems are therefore often seen in the countryside where it is common to have the system installed in an annex. Here use of an existing DHW tank as a separate DHW tank (sp.) in the main building is sometimes seen as a solution.

2 2.2 Biomass boilers Distinctions should be made between systems with manually fired biomass boilers for firewood and systems with automatically fired boilers for wood pellets or wood chips. The project is mainly dealing with systems with manually fired biomass boilers, but also systems with automatically fired boilers are taken into consideration for reference purposes. 2. Auxiliary energy sources For systems with a manually fired biomass boiler an additional auxiliary energy source (apart from solar heating system and biomass boiler) is recommendable, but dispensable. For systems with an automatically fired biomass boiler it is, however, necessary to have some kind of auxiliary energy source, in order to be able to avoid use of the biomass boiler in summer. Examples of auxiliary energy sources are electrical heaters (in the DHW tank), oil burners, or decentralized external water heaters. Often the old existing oil boiler is kept as an additional auxiliary energy source, when a new biomass boiler is installed. One of the approved manually fired biomass boilers on the Danish market has an integrated oil burner, which can be used, as additional auxi l- iary energy source (boilers in system S and MS2). 2.4 Summary of experiences from inspected systems Table 1 shows collector areas, storage volumes, storage types, DHW volumes/types, types of auxiliary energy sources, and boiler types for the 12 inspected systems. A coll [m²] V storage [m³] Inspected systems Storage type V DHW [m³] Aux. type Boiler type S sub no manually S sp. oil manually S sub no manually S sub no manually S sub oil manually S sub oil manually S manually S auto MS sub EWH 2 manually MS sub oil manually MS sp. EWH 2 auto MS EHX 4 EH manually Table 1: Overview of inspected systems. 1 System has been expanded with an extra storage tank since inspection 2 Electric Water Heater (external) Only rough measurements of temperatures and quantities of energy 4 External Heat Exchanger Electric Heater (in storage) As shown, not all the information is available for all systems. For systems marked with an M measurements have been carried out. Some of the problems that occurred at the inspected systems were: Oversized biomass boilers compared to heat demand Undersized storage volumes compared to boilers Safety problems with regards to boiler installation Over- or undersized collector areas Insufficient insulation The problems with oversized biomass boilers presumably arise from the fact that most biomass boilers on the Danish market, except for a few of the newest models, are oversized compared to the heat demand of typical Danish single-family houses. Also, there were several examples of stop-gab solutions, cobbling together old and new components without paying sufficient attention to the overall solution. In some cases a new biomass boiler had been added to an existing solar heating system, (possibly together with an extra storage tank). In other cases a new solar heating system had been added to an old biomass boiler. Typically, such old biomass boilers didn t work well and were oversized or the storage volumes were too small to contain the surplus energy from one charge (with full magazine). Examples of bad installation practice were an old boiler with lateral burn-up and an illegal installation causing safety problems. There were a few examples of over- and undersized collectors, causing either overheating problems or not allowing the biomass boiler to be switched of during summer periods. In general, many systems were insufficiently or haphazardly insulated, and together with many piping connections this must have caused great heat losses.. MEASUREMENTS CARRIED OUT At first only three of the inspected systems were chosen for monitoring (MS1, MS2, and MS). Later in the project period a new system (MS4) inspired by already gained experience was developed and installed. This system has only been equipped with calorimeters and thermometers for manual reading. The three systems MS1, MS2, and MS have been monitored from January or March 1999 until January 2 incl. The monitoring have been carried out by the Test Station for Solar Heating Systems at Danish Technological Institute. Equipment for registration of temperatures, flow, power consumption and other essential factors was installed. The measuring data were logged every th second and three-minute average values were registered in files. The three systems are described in section 4 and the test points are indicated on the hydraulic schemes in Figure 1. The measurements comprised were:

3 MS1 MS2 l TH TH 7 l 1 l 18 l T6H T6L M6 MS MS4 Stoker Wood stove TH TH 16 l l 7 l Figure 1: Hydraulic schemes of the measured systems MS1, MS2, MS, and MS4. Temperatures of storage tanks, boilers and solar heating loops Running times and patterns of use Flow, energy flow, boiler output and solar output With regard to an estimation of the environmental impact of the three systems, separate measurements of emissions and efficiencies of the biomass boilers have been carried out as a supplement to the fixed measuring program. These measurements have only been carried out on one spring day. All together, the described measurements have provided the background for evaluation of following quantities: Operational and load profile of collector and boiler Solar performance Average output of biomass boiler Annual and seasonal efficiencies 6 Since November 1999 the owner of system MS4 has performed and reported weekly readings of energy quantities and temperatures. 6 Registration of current biofuel consumption (and thus calculation of boiler efficiency) has only been possible for system MS with stoker. 4. MEASURED SYSTEMS The four systems, for which measurements have been carried out, represent quite different but interesting system designs. Hydraulic schemes of the four systems are shown in Figure System MS1 System MS1 is characterized by having two storage tanks (in parallel) - one of them a tank-in-tank. The system is installed in an annex. The DHW is after-heated in an electric water heater in the main building. The solar heating system has an open expansion tank and collectors are facing west. Generally the system appears as being well insulated. The nominal boiler output is 6 kw. Depending on the density of the firewood used, a total storage volume of m³ would be optimal. Current volume is 1.7 m³ and thus quite suitable. There are no shunts in the heating system. System MS1 should not be considered as an example of a typical Danish Solar & Biomass system design. 4.2 System MS2 System MS2 is a typical Danish Solar & Biomass system design in the way that the system has been built together

4 gradually. It has been continuously extended, renovated and adjusted by the owner himself and has become quite complicated. Also, the small storage tank the tank in tank of. m³ - is very common. The two storage tanks are connected in parallel. DHW is preheated in an internal heat exchanger in the bottom of the big storage tank and further heated in the submerged DHW tank of.12 m³ in the small tank. Before use the DHW is finally after-heated in another internal heat exchanger in the top of the big storage tank. Solar heating is delivered to the bottom of the small tank via an internal heat exchanger. System MS2 is the only system with circulation piping for DHW. The piping is connected to the internal heat exchanger at to the top of the big storage tank. Radiator heating system and floor heating system have shunts. Heat for the radiators is taken from the top of the small tank and returned to the bottom of the big tank. The floor heating system is connected only to the big tank. Nominal output of the biomass boiler is 24 kw and the total storage volume of 2. m³ is considered quite suitable. The boiler has an integrated oil burner for backup and summer operation. 4. System MS System MS is an example of a Solar & Biomass system with an automatic boiler for wood pellets. A specific company in Denmark markets the solar design as an overall solution. The storage tank is a special tank with a built-in drainback tank of.17 m³. The system is installed in an annex. As a part of the system design, the existing hot-water tank and the existing electric water heater in the main building are retained for DHW preparation. The existing DHW tank is heated by the central heating loop by an internal heat exchanger. A detachable stoker has been mounted on a 2 years old existing boiler (cast iron) with a nominal output of 48 kw, constituting severe over-capacity. During summer periods the boiler loop can be switched of manually. 4.4 System MS4 System MS4 is a newly developed system with a special designed storage tank constituting the heart of the system. It has an external heat exchanger for preparation of DHW. A roomheater (wood stove) fitted with a boiler is placed in the main building. Nominal space heating output, provided as radiation and/or convection, is kw. Nominal water heating output is also kw. The water heating output is transferred to the storage tank by means of pulsating operation. All inlet devices in the storage tank are designed for maintenance of good stratification in the tank. The tank is supplied with an electric heater (6 kw), which is prima r- ily used to avoid frost damages during absence in winter periods.. RESULTS FROM MEASUREMENTS When dealing with measurement results, methods of fractional energy savings are not suitable. For systems combining solar and biomass heating - by means of a common storage tank - distinction between solar and biomass contributions is debatable. It must at least be stated how the distinction is made and how the heat losses from the storage tank are distributed. However, in order to give a survey of energy demands covered by solar heating, and energy demands covered by biomass heating distinctions will be made here..1 Energy Survey For each of the three monitored systems MS1, MS2, and MS, a complete energy survey is shown in Figure 2 Figure 4. Energy contributions are summed up during the monitoring period. Missing data appear as vertical blanks. As shown the energy demand is, of course, highest for system MS2, which provides a multifamily house. What cannot be seen from Figure is that the energy demand for circulation of DHW constitutes 7 % of the total energy demand for DHW. In Figure 2 Figure 4, the difference between sum of energy demands for DHW and space heating and the total energy output from boiler loop, solar heating loop, and electric water heater (EWH) illustrates the heat losses of the system. As shown, the heat losses of system MS1 are very small, whereas the heat losses of system MS2 are very high. The energy contribution from the solar heating loop of system MS2 only makes up for half of the total heat loss. Figure 2 shows that the solar performance of system MS1 is poor. This is partly due to the fact that the collectors are facing west and partly because of problems with the open expansion tank causing severe heat losses. However, as the heat losses from the system are also poor, the output from the solar heating loop actually covers a small part of the energy demand. [kwh] Energy Survey, System MS time [weeks] DHW+EWH Space Heating Boiler+Solar+EWH Boiler+EWH Boiler Figure 2: Monitored energy survey of system MS1. 4

5 [kwh] Energy Survey, System MS time [weeks] DHW Space heating Boiler+Solar Boiler Figure : Monitored energy survey of system MS2. 2 Frequency [%] Inlet Temperature, Solar Heating Loop >7 Inlet Temperature [ C] MS1 MS2 MS Figure : Frequency distribution of forward temperature in solar heating loop. [kwh] Energy Survey, System MS The high level of inlet temperatures in system MS2 is found mainly to be due to the way of connecting the circulation piping for the DHW to the storage tank time [weeks] DHW+EWH Space heating Boiler+Solar+EWH Boiler+EWH Boiler Figure 4: Monitored energy survey of system MS. For system MS the output from the solar heating loop covers both heat losses as well as a considerable amount of the energy demand..2 Operation conditions, solar heating system One operation condition that has a great influence on the behavior of the solar heating part of the system is the temperature level. Some of the parameters that might influence the forward temperature in the collector loop are storage design (level of stratification), control strategy and in particular design and operation of the space heating loop. For each of the three monitored systems, Figure shows frequency distribution of forward temperature in solar heating loop, based on one-hour averages of registered temperatures. For system MS1 the forward temperature of the solar heating loop (secondary site) is centered about 2-4 C, not exceeding 6 C. System MS has a generally higher collector inlet temperature, though varying a lot. About 2 % of the collector loop running time the forward temperature is between and C. In system MS2 the general forward temperature is even higher (too high). More than 2 % of the collector loop running time the inlet temperature is between 6 and 6 C. 6. Operation conditions, biomass boiler For systems with automatically fired boilers for wood pellets or wood chips, it is of great importance that the boiler nominal output (at full load) does not exceed the maximum output demand in winter periods. During summer periods the hot water demand often only equals -1 % of the boiler nominal output. Such an operating method reduces the efficiency and increases the emissions the efficiency is typically reduced with 2- % compared to the efficiency at nominal output (Lars Nikolaisen, 1999). When combining a solar heating system with an automatically fired boiler, it is therefore a good idea to design the solar heating system so that it can fulfill the hot-water demand during summer periods (maybe by using an electrical heater or another kind of auxiliary energy source). Similarly, a principle rule for systems with manually fired boilers for firewood is that the boilers only have an acceptable combustion at full load (Lars Nikolaisen, 1999). An exception, however, is boilers with oxygen control they may be used down to % of full load without reducing efficiency or increasing emissions. Boiler Output [W] Average Boiler Output, System MS1 March May June Aug. Sept Feb. April July Oct.. Time Nov. Dec. Figure 6: Monitored boiler output (1-hour average) of system MS1. Period: January January 2. Jan % of Nominal Boiler Output

6 Figure 6 shows 1-hour average values of boiler output for system MS1over the measuring period. Over a summer period of 12 days, the boiler was not used. The rest of the year the average boiler output was between kw and 4 kw (corresponding to -111 % of nominal boiler output). For all three systems the boiler output have generally been far below 1 %. In average system MS1 has the highest boiler output in percentage of the nominal boiler output. In system MS2 the boiler output was generally lower and with a more evident seasonal dependency. As shown in Figure 7 the boiler was used throughout the year in system MS2. Due to this fact it has not been possible to close down the big storage tank in summer, which is also a reason for the considerable and significant heat losses of system MS2. Boiler Output [W] Average Boiler Output, System MS2 March May June Aug. Sept Feb. April July Oct.. Time Nov. Dec. Jan Figure 7: Monitored boiler output (1-hour average) of system MS2. Period: March January 2. System MS was the worst system with regard to operation conditions. The boiler output was always below 2 % of nominal output. This is most unfavorable for an automatic boiler, which should not be running at less than % of nominal output. Most of the time the boiler efficiency was between 4 % and 6 %. See Figure 8. Over a summer period of 16 days, the boiler has not been used. Efficiency [%] Boiler efficiency (4-hour averages) March May June Aug. Sept Nov. Dec. Feb. April July Oct. Jan.. Time Figure 8: Calculated boiler efficiency (4-hour average) of system MS. Period: January January 2. There has been no significant seasonal dependency. The two peaks in March and November are due to thorough % of Nominal Boiler Output cleaning of both firebox and fire tube. The owner claims to have cleaned the firebox regularly, but admits only to have cleaned the fire tube in March and November. 6. GUIDELINES As a part of the project the following guidelines for good system design have been prepared. 6.1 Heat losses Installation design must be with as few tube connections and as good insulation as possible. Furthermore when using two storage tanks it should be possible to disconnect the one in the summer period. It is important to avoid heat losses - especially in the circulating piping, which demands high temperatures. If possible the circulation should be turned off or the system should be carefully dimensioned to the actual demand, which then in many cases has to be specially monitored or estimated. It would be valuable to analyse heat demands and cut down unnecessary losses in advance. However, if collectors are cheap enough, an enlarged collector area could also meet unnecessary demands. 6.2 Temperatures in system Low inlet temperatures for the solar collector could be achieved by insuring: 1. Low return temperatures from heating system (radiators) 2. Utilisation of the cold water temperature (DHW). Good thermal stratification in storage tanks 4. Excess storage volume with low temperatures when the boiler loads the storage tank. Re 1) Monitoring shows that in most cases the user waits to fire up the boiler until the temperature in the house has decreased below comfort temperature. If there is no shunt in the heating loop this means that when starting the boiler the radiator thermostats are wide-open and flow and return temperatures are high. From the measurements it is seen that this causes all temperatures in the system to rise considerably, then causing reduced efficiency of the collectors. Suggestions could be: To have efficient stratifiers in the tanks so that high return temperatures will rise to the top of the tank To have automatic valves directing the high temperature flows into the top of the tank To have limited allowed flow, which however would delay the procedure of bringing the house temperature back to normal Re 2) In the monitored systems the influence of the cold DHW temperature on the collector inlet temperature was

7 not very high. More sophisticated design could improve this. Re ) The monitored systems seem to have good thermal stratification except for what is mentioned above. Inlet tubes and flows should be designed to avoid mixing in the storage tank. Re 4) The monitored systems seem to have good storage capacity to contain both heat from the boiler and the collector. Storage volume should be designed to contain the heat from one charge of the boiler plus heat from the collector. In systems with two storage tanks one should be disconnected in summer. 6. Biomass boiler Manually fired boilers should always be used at full load (except from boilers with oxygen control) For automatic boilers it is of great importance that the boiler nominal output (at full load) does not exceed the maximum output demand in winter. Regular cleaning of both firebox and fire tube has a great influence on the efficiency of the boiler. 6.4 Generally In order to avoid complicated systems that don't function well it is recommended to: inspired by already gained experience. The work carried out in this project is the first step on the way to better Solar & Biomass system designs in Denmark, but there is still a lot of work to be done. REFERENCES Book: Nikolaisen L. and others (1999). Wood for Energy Production, Technology Environment Economy, 2 nd edn. pp. -6. The Centre for Biomass Technology, Danish Technological Institute, Århus. Report in Press: Overgaard L.L. and Ellehauge K. (2). Erfaringer fra målinger på kombinerede solvarme- og biobrændsels- Danish Technological Institute, Århus. Report: Ellehauge K. Møller T.K. Jacobsen H.J. (1999). Aktive solvarmeanlæg med større dækning af husets samlede varmebehov, pp. 1-. Solar Energy Center Denmark, Danish Technological Institute. NOMENCLATURE A coll : Collector area [m²] V: Volume [m³] Keep the system design as simple as possible Think in overall solutions 7. CONCLUSIONS The 12 inspected Danish systems represent both welldesigned and well-functioning small Solar & Biomass systems as well as poor system designs. Some of the faults experienced were oversized and undersized components and insufficient insulation. Also, there were several examples of stop-gab solutions resulting in very complicated systems with a lot of piping connections, great heat losses and a great risk of making operation mistakes. Monitoring of three of the systems over a period of almost a year have shown great differences with regard to operation conditions, heat losses from installations, solar contributions, solar gains as well as number of days the biomass boiler can be turned of each year. In general the solar gains of the three systems were not very large. Generally, there is a need for better overall solutions with components adjusted to the actual demands and boundary conditions. As a part of the project such a new system with one storage tank only has been developed - Solar collector Flat plate heat exchanger Heat exchanger Heat storage DHW tank Drain-back tank Variable power auxiliary boiler Fixed power auxiliary boiler Electric heater Heating floor Radiators Shower Heating fluid/dhw Antifreeze fluid Pipes crossing without connection Pipes crossing with connection 4-way valve -way valve 2-way valve Thermostatic valve Pump Temperature sensor