MINIMISING THE RISK OF WATER HAMMER AND OTHER PROBLEMS AT THE BEGINNING OF STAGNATION OF SOLAR THERMAL PLANTS- A THEORETICAL APPROACH

MINIMISING THE RISK OF WATER HAMMER AND OTHER PROBLEMS AT THE BEGINNING OF STAGNATION OF SOLAR THERMAL PLANTS- A THEORETICAL APPROACH Wolfgang Streicher Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25, Graz, A-8010 Austria Phone:+43-316-873-7306, Fax: +43-316-873-7305, E-Mail: streicher@iwt.tu-graz.ac.at Abstract Solar plants are increasingly used not only for hot tap water heating but also for the assistance of space heating. These plants produce much more energy in summer than needed, which often results in stagnation. Because of stagnation-temperatures of today s selective collectors up to 200 C, the collector fluid evaporates. In several plants a high noise level and a vibration of the plant during this evaporation phase is reported. This is due to the occurrence of water hammers in the system, when liquid collector fluid passes areas where the fluid was already evaporated and superheated. The remaining vapor bubbles deflate rapidly and the liquid phases collide with high velocity, which results in a rapid pressure increase. This paper describes the theory of condensate-induced water hammers and conditions of solar plants, under which this can happen. A simulation model for the evaporation phase of the collector is presented to give a deeper understanding about the influences of solar radiation, the size of the tubing, and the size of the expansion device on the process of the evaporation. Three hydraulic layouts of the collector area are discussed for there possibilities producing water hammers. 1. INTRODUCTION Solar plants are increasingly used not only for hot tap water heating but also for the assistance of space heating. These plants produce much more energy in summer than needed. Additionally solar plants in single and multifamily houses for hot tap water production are often oversized for the summer demand. If there is no other heat sink available this often results in stagnation of the collectors. Because of stagnation-temperatures of today s selective collectors of up to 200 C the collector fluid evaporates. In several plants a high noise level and a vibration of the plant during this evaporation phase is reported (Hausner, Fink, 2000). In one big Austrian plant the breaking of an 11 bar safety device was reported although the safety valve was adjusted with 6 bar (Holter, 1998). For solar plants mainly condensate-induces water hammers occur. Condensate-induced water hammer is caused by rapid condensation of steam by subcooled water. It can occur in several different ways. 2. THEORY OF WATER HAMMERS Water hammer, also known as steam hammer, is a pressure or momentum transient in a closed system caused by a rapid change in fluid velocity. It is classified according to the cause of the velocity change. The types of water hammer include the following. Valve-induced water hammer Void-induced water hammer Flashing-induced water hammer Steam-propelled water slug Condensate-induced water hammer. Fig.1 Principle of condensate-induced water hammer in a horizontal tube (DOE/EH-0560, 1998) The most common condensate-induced water hammer is caused by steam flowing over subcooled water. The

steam flow causes ripples in the water surface. If one of these ripples touches the top of the pipe, it can momentarily seal off a pocket of steam, which then condenses and collapses, causing a pressure wave. This is shown in Fig 1. In solar plants water hammers are not reported directly, but sometimes noise problems during stagnation occur, which are probably due to water hammers. 3. CIRCUMSTANCES FOR THE OCCURANCE OF WATER HAMMER IN SOLAR PLANTS In solar plants principally four situations are known, in which water hammer can occur: During the first filling of the collector when the sun is shining. In this case subcooled collector fluid is pumped or running (if the tubing goes downwards) over superheated collector areas. The fluid evaporates at the superheated tube-walls of the collector. When the vapor bubbles are moving into the subcooled water they are collapsing again and water hammer can occur. This is being often reported by the installers (i.e. Holter, 1998). It can be avoided by filling the plants only when the sun is not shining. This situation only occurs seldom (during first fill up or after repairs of the plant). Stagnation of the collector, because the heat demand of all available heat sinks is already satisfied. This situation can occur once a day during the summer and is therefor analyzed in detail in the following. Startup after stagnation during sunshine. With normal collector pumps this situation cannot occur, because the pumps are not able to overcome the static pressure difference of the fluid in the collector. In the last years several pump-producers began to develop gear-pumps for solar plants in order to have higher efficiencies. These pumps can operate with pressure differences up to 2,5 bar and can therefor easily perform a startup of a stagnated and evaporated collector by pressing the steam out of the collector. In this case the same conditions occur as during the first filling. To avoid this condition, the collector-control should only allow a startup, when the collector temperature is below the evaporation temperature. Condensation of evaporated collector fluid when the solar radiation decreases. If the condensate can flow into the collector there may also situations occur, in which steam is completely surrounded by liquid collector fluid and water hammer are formed. 4. STAGNATION OF COLLECTOR Several phases can be seen in the during the stagnation phase. Heating of the collector fluid below boiling temperature Evaporation of the collector fluid Superheating of steam in collector Condensation of steam in tubes after the collector is at thermal equilibrium Recondensation of fluid in the collector when the solar radiation decreases They are described in the following 4.1 Heating of the collector fluid below boiling temperature When a collector goes into stagnation, normally the sun is shining. Therefor the collector temperature is rising until the evaporation temperature is reached. The pressure slightly increases due to the expansion of the fluid. 4.2 Evaporation of the collector fluid When the evaporation temperature is reached, the collector fluid starts to evaporate at the top of the collector, because of the lowest system pressure at this point. To understand what is happening during the evaporation one has to analyze the hydraulic layout of the plants. In principle two different extreme situations have to be analyzed. In realty there will often be a situation between these two extremes. Case 1: U-tube formed collector area connected to the tubing at the bottom. Liquid collector fluid is pressed out of the collector into the connecting tubes. This is the case for collector-areas connected as shown in Figure 2 The inlet and outlet connections are at the lowest point of the collector. Normal operation Start of stagnation Liquidphase not oscillating; no water hammer Fig. 2 Hydraulic collector flow scheme pressing liquid out of the collector during evaporation (case 1) Case 2: U-tube formed collector area connected to the tubing at the top. Steam is pressed out of the collector. This situation occurs in collector areas as

shown on Figure 3 where the collector inlet and outlet is located at the highest point of the collector. Oscillating liquid phase may creates water hammer Fig. 3 Hydraulic collector flow scheme pressing steam out of the collector during evaporation (case 2) Case 1, evaporation phase Most of the collector fluid in Fig. 2 is evaporated within seconds up to a few minutes depending on the solar radiation and the collector type. The liquid volume of the collector is pressed into the connecting tubes and the expansion device. Therefor the pressure rises sharply. At the end of the evaporation the pressure rise smoothes, because the boiling area of the collector decreases and most of it is already superheated. After the volume of the collector is completely filled with steam the pressure rise nearly stops. Whether saturated fluid enters the expansion device or not is dependent on the collector volume compared to the volume of the connecting tubes, pumps, valves etc. No steam is entering the tubes with this collector configuration. Case 2, evaporation phase In Fig. 3 the situation is different. In the evaporation phase, the steam is pressed out of the collector into the connecting tubes. Most of the liquid in the collector has to be evaporated and the volume of produced steam is about 500 times the volume of the situation described before. The steam is superheated in the collector when flowing from the evaporation zone to the collector outlets. This steam is, to a minor part, cooled down and condensed in the insulated tubes. The major part condenses in the heat exchanger of the store. With external heat exchangers steam can also occur on the secondary side (Hausner, Fink, 2000). The pressure rises sharply in the beginning, when the vapor is filling the tubes from the collector to the heatexchanger. In this period the danger of water hammer is given, because steam and liquid fluid can be mixed in this highly transient phase. After the tubes are filled with steam the pressure rises only smoothly due to an increasing steam volume in the collector and an increasing superheat. The amount of steam produced in the collector drops, due to the decreasing area of the evaporation and increasing collector temperature with increasing thermal losses to the environment. The area for superheat is increasing. The steam volume in the tubes is constant, as all surplus steam is condensed in the heat exchanger (if sufficiently dimensioned). In addition to the collector also the tubes are filled with steam. Therefor the pressure is much higher than for the situation in case 1, where only the collector is filled with steam. At the end of the evaporation phase there will be more steam being condensed in the tubes than produced by the collector and the pressure drops. 4.3 Superheating of the steam in the collector This phase occurs in both situations described above. Although there are differences. In case 1 steam is entering the tubes according to increasing volume of the superheated steam in the collector. This steam is being condensed in the tubes. The pressure rises slightly because of the increasing steam volume. In case 2 superheat occurs already in the evaporation phase. If the amount of steam being pressed out of the collector is less than the amount being condensed in the tubes the total amount of steam is decreasing and therefor the pressure decreases. The tubes are being filled slowly with condensed collector fluid (recondensation). 4.5 Condensation of steam in tubes after the collector is at thermal equilibrium After the thermal equilibrium of the collector is reached in case 1 (thermal losses are equal to solar gains), the tubes are completely filled with condensed collector fluid and start cooling down to ambient temperature of the tubing. The pressure in the systems decreases slightly and reaches steady value. In case 2 condensate can enter the collector when the tubes are completely filled with condensed collector fluid and run down the collector tubes. As the steam in the collector is superheated, this condensate is evaporated rapidly, steam is pressed into the tubes and is again slowly condensed. Therefor pressure transients can occur. In Case 1 no liquid can enter the collector. 4.6 Recondensation of fluid in the collector when the solar radiation decreases When the solar radiation decreases and the temperature in the collector drops below boiling temperature recondensation in the collector starts. In case 1 this happens slowly and condensate fills the collector from bottom to the top. In case 2 liquid collector fluid is

Collector sucked into the collector and flows down to the bottom. In this case the possibility of steam bubbles surrounded by liquid is given, therefor again water hammer can occur. The main result of the above analysis is that a case 1 layout is always superior to a case 2 layout. Unfortunately the common layout is case 2 and sometimes there is even no possibility to connect the inlet and outlet at the bottom of the collector to the connecting tubes going straight downwards. In this case the connecting tubes can be mounted parallel to the collector flowing first upwards without being exposed to solar radiation and then flowing downwards to the store where it is appropriate. the U-tube of the collector underpressure can occur on the blocked side sucking liquid collector fluid into the collector. Blocking the collector is also hindering the evaporation for case 1. If the fluid can be pressed out of the collector only in one way half of the collector fluid has to be evaporated resulting in similar problems as in case 2. The lesson learned from case 3 is that the hydraulic layout should allow the collector being drained in both directions. Fig. 5 shows a hydraulic layout of pump, expansion device and one-way valve allowing this and providing a constant suction pressure of the pump. 4.7 Worst case scenario for water hammers (case 3) An even bigger possibility of the occurrence of water hammer is given with a hydraulic layout shown in Fig. 4. This case is similar to the case 2 described above but steam can only leave the collector in one direction, because the other direction is blocked by a one way valve. During the evaporation phase the steam of the blocked side presses liquid collector fluid to the non blocked side. There it passes areas of the collector that are superheated and flows into the connecting tube that is also filled with superheated or saturated steam. In this areas the occurrence of water hammer is highly probable. Filling valve Valve Manometer + thermometer Collector pump Valve Filling valve Safety valve One-way valve Expansion from both sides in expansion device possible Discharge valve Expansiondevice oscillating liquid-phase may creates water hammer Condensation Fig. 5 Hydraulic flow scheme of pump, one-way valve and expansion device allowing the flow from both collector sides Fig. 6 shows a common hydraulic layout for big solar collectors (case 4) that should work similar to case 1. The difference is, that no U-tube is formed in the collector. Therefor the connecting tube is filled with steam to the same height as the collector during the evaporation phase (communicating vessels). Fig. 4 Hydraulic collector flow scheme of a tubing that is blocked on one side (case 3) A part of the liquid drains back into the collector passing again steam filled areas. If the water starts oscillating in If the tubes are not fully horizontal or falling in the direction of the collector inlet, not all liquid will be pressed out of the system in the beginning of the evaporation phase. Then case 2 or even case 3 effects can locally occur during the final evaporation of the liquid that is left in the collector tubes.

In the thermal equilibrium phase condensation takes place in the tubes. When the liquid content of the connecting tubes increases, liquid is also pressed into the collector bottom and evaporates due to the communicating vessels. Therefor in the thermal equilibrium phase a slight natural convection takes place in the collector loop. As this happens slowly, no water hammer should occur. Normal operating pressure 150 kpa All steam reaching the heat exchanger was assumed to be condensed. The collector fluid was assumed to be propylene-glycol. As no thermophysical properties of propylene-glycol in the gaseous phase were available, the values of water (steam) were used. The calculation is performed in several steps: Normal operation Start of stagnation Liquid- phase not oscillating; no water hammer Heating of the collector fluid below boiling temperature (ref. 4.1) Evaporation of collector fluid (ref. 4.2) Superheat of collector fluid (ref. 4.3) Recondensation of collector fluid in connecting tubes (ref. 4.4) Condensation Fig. 6 Common hydraulic layout for large area collectors (case 4) Fig. 7 shows the steam production of the collector and the condensation in the tubes. In Fig. 8 the pressure rise of the system in that phase is shown. All steam that stays in the system causes collector fluid entering the expansion device yielding a pressure rise. The tubes are filled with steam within seconds. In order to expand this time the solar global insulation was chosen with 500 W/m 2 for the calculations of Fig. 7 and Fig. 8. 5. SIMULATION OF THE STAGNATION OF SOLAR COLLECTORS In order to analyze the different stages of the stagnation period a simple simulation model calculating case 2 was written using the software package EES (Klein, 1999). As a reference case the following values were chosen: Collector Global insulation on collector area 800 W/m 2 Net. collector Area: 15 m 2 Thermal length (length of tubes in series): 10 m Sunstrip absorber Transmittance*Absorption (τα) of collector 0.8 Overall heat loss coefficient of coll. 3.5 W/m 2 K Heat transfer coefficient liquid-wall 500 W/m 2 K Heat transfer coefficient boiling-wall 2000 W/m 2 K Heat transfer coefficient superheat-wall 50 W/m 2 K Ambient temperature 30 C Connecting tubes Overall length Diameter Insulation thickness Insulation thermal conductivity 20 m 0.016 m 0.01 m 0.04 W/mK Fig. 7 Steam production and recondensation in tubes during the steam-filling of the tubes (case 2, 500 W/m 2 global insulation) The pressure rise, of course, induces a temperature rise in the collector (boiling temperature). Therefor the heat losses of the collector increase and the steam production decreases. In the same way the condensation in the tubes increases with increasing temperature of the steam pressed in the tubes. No superheat of the steam produced in the collector occurs during that phase. Expansion device Media Nitrogen Volume 0.02 m 3

fast pressure rise due to the steam filling the tube can be seen. Then the pressure rise smoothes down because of the decrease of the steam production in the collector. This is due to the decreasing boiling area of the collector (area with liquid collector fluid) and the increasing heat losses of the collector to the ambient. Fig. 8 Steam in tubes and pressure rise during the steamfilling of the tubes (case 2, 500 W/m 2 global insulation) Fig. 9 shows the collector outlet temperature and the boiling temperature over the period of the first 15 minutes. When there is steam produced in the collector, it is forced to flow out of the collector into the tubes. This steam is superheated during its flow in the collector. As bigger the steam volume in the collector gets, as longer the steam gets energy and the superheat increases. With the simple model used, the stagnation temperature is reached after about 10 minutes. In realty this can not happen, as long as there is steam flowing. Nevertheless the superheat will be close to the stagnation temperature. Fig. 10 Steam production in collector and condensation in tubes and the resulting saturation temperature from stagnation start to recondensation of the tubes (case 2, 800 W/m 2 global insulation) Fig.11 System pressure and Nitrogen content of the expansion device from stagnation start to recondensation of the tubes (case 2, 800 W/m2 global insulation) Fig. 9 Steam in tubes and pressure rise during the steamfilling of the tubes (case 2, 800 W/m 2 global insulation) The saturation temperature and therefor the system pressure increases only slightly because it was assumed that all steam that reaches the heat exchanger is condensed completely. As the tubes are already filled with steam, only the slowly increasing steam volume (decreasing of the fluid level) of the collector gives additional steam volume. Figs. 10 and 11 show an expanded timeframe of the stagnation process. In the beginning of stagnation the After 2.5 hours there is more steam condensed in the tubes than produced in the collector. Therefor the tubes are filling with liquid collector fluid again (recondensation). The system pressure decreases. It will reach a steady value, when the tubes are filled with liquid collector fluid completely. This takes a long time and will not be reached during full solar insulation in reality. Anyhow, the system pressure that would be reached is the maximum system pressure, that would occurs, if a case 1 layout would have been chosen. The tubes are filled with liquid and the collector is fully superheated. The maximum pressure rise in case 1 will therefor be much

smaller than the one in case 2. This is, of course, depending on the volume of the connecting tubes. Figs. 12, 13 and 14 show parameter variations of the simulated system and the resulting system pressures from stagnation start to recondensation of the tubes. In Fig. 12 the solar global insulation was varied from 500 to 800 W/m 2. With decreasing insulation the maximum system pressure decreases, because of smaller steam production and lower stagnation temperature. The recondensation of the tubes starts later, because it takes more time to increase the pressure. Below 450 W/m 2 global insulation no evaporation in the collector occurs with the starting pressure of 150 kpa. presure [kpa] 450 400 350 300 250 200 150 0 3600 7200 10800 14400 18000 time [s] above, the tube diameter should be kept small in order to decrease the overall possible steam volume and therefor system pressure in the system. Smaller tubes can condense less steam. Therefor the refilling of the tubes with condensate starts later for smaller tubes. pressure [kpa] 450 400 350 300 250 200 150 0 3600 7200 10800 14400 18000 time [s] 0.012 m 0.014 m 0.016 m 0.018 m Fig.14 System pressure from stagnation start to recondensation of the tubes with varying connecting tube diameter (case 2 800 W/m 2 global insulation) 800 W/m² 700 W/m² 600 W/m² 500 W/m² Fig.12 System pressure from stagnation start to recondensation of the tubes with varying solar insulation (case 2) Fig. 13 shows the dependency of the system pressure on the volume of the expansion device. An increasing volume decreases the maximum system pressure. pressure [kpa] 500 450 400 350 300 250 200 150 0 3600 7200 10800 14400 18000 time [s] 18 l 20 l 22 l 24 l Fig.13 System pressure from stagnation start to recondensation of the tubes with varying volume of the expansion device (case 2, 800 W/m 2 global insulation) In Fig. 14 the effect of different connecting tube volumes varied with the tube diameter is shown. As mentioned 6. CONCLUSIONS During stagnation of a solar collector several phases can be seen. After a temperature rise in the liquid state to boiling temperature with a slight pressure rise, evaporation in the collector causes a rapid pressure increase. In this phase water hammer can occur, if the hydraulic layout of the collector is not appropriate. When the collector is built as an U-tube open to the bottom (case 1) the pressure rise slows down when the steam in the collector has pressed out all liquid. A little pressure rise still occurs due to superheating in the collector. If the layout is like an U-tube open to the top (case 2), the pressure rise slows down when steam reaches the heat exchanger and condenses again. The pressure still rises due to an increasing steam volume in the collector. Also in this case water hammer may occur. When the steam volume flow slows down and there is more steam condensed in the tubes than produced by the collector the pressure drops. The recondensations finishes, when the collector is at thermal equilibrium. When the solar radiation decreases, the collector starts condensing and again water hammer may occur in case 2. Therefor case 1 (or case 4) shall be preferred compared to case 2. Anyway the collector fluid should be able to be pushed out of the collector in both ways to the expansion device (avoid case 3).

A simple simulation tool for case 2 was developed. All phases from heating up of the collector fluid until refilling of the tubes with liquid collector fluid can be calculated. For case 2 the expansion device shall be designed to collect not only the fluid pressed out of the collector but also the tube volumes. The tubes should be made as small as possible. REFERENCES Doe/EH-0560 (1998) Water Hammer, NFS Safety Notices, Issue No. 98-2, Department of Energy, Washington. Klein, S. (1999) EES Engineering Equation Solver, F- Chart Software, Madison, WI. Holter Ch. (1998) Personal information about the damage after a stagnation of the solar plant Eibiswald, Austria. Hausner, R., Fink, Ch. (2000) Untersuchungen zum Stagnationsverhalten von Anlagen zur solaren Heizungsunterstützung, In Proceedings of Solare Raumheizung, 3. March, Graz, Austria, Abeitsgemeinchaft ERNEUERBARE ENERGIE, Austria. Duffie J.A. and Beckman W.A. (1991) Solar Engineering of Thermal Processed, 2 nd edn. pp. 54-59. Wiley Interscience, New York.