Maximizing the Utilization of Stored Heat

After producing heat with the highest possible efficiency of the boiler and the solar collector and keeping the heat losses as small as possible, it is also important to use the potential of the heat capacity of the heat store in the best way. Since the volume after installation is fixed, only the maximum and the minimum temperatures which occur in the tank can influence the heat capacity. A high heat capacity of the tank has the following effects in the system:

• The start/stop frequency of the boiler is reduced.

• The amount of heat from the solar collector that can be stored in the solar tank is increased.

• Due to high heat capacity based on advanced operation strategy the tank size can be reduced, which also reduces costs and space requirement.

To minimize the temperature in the tank means that the temperature difference between the forward temperature and the return temperature during discharge of the tank should be as large as possible. Therefore, also the process of discharging the tank should take place according to the “low flow” principle with the main goal to achieve lowest possible return temperatures.

Mainly in the period with the potential of strong irradiation and therefore high solar gain, it is important to be able to charge the solar tank as much as possible with solar energy. Therefore, it is also important to minimize the temperature and the volume of the auxiliary volume in the tank as much as possible. Especially for small solar tanks this can have a quite large effect. It is a big difference, if in a 300 liter tank about 100 liter auxiliary volume is kept at a set temperature of 60 to 65°C or not.

If the solar tank is a hot water tank, due to the risk of lime problems very often the controller is set to switch off the solar pump between 60 and 70°C. If the solar tank is a space heating tank, the only limitation is the boiling temperature and the maximum temperature the material of the tank and the insulation can withstand. Therefore, in a typical space heating tank made of steel, temperatures up to 100°C are possible.

2.5.1 Thermal Stratification

In order to achieve the goals of best possible utilization of the energy stored in the tank, one of the most important tasks is to build up and keep good thermal stratification in the tank in the best possible way. The optimum would be, at any time, to have all useful heat stored in the top part just exactly at the set temperature and with a sharp border, the rest of the tank volume should have the lowest temperature occuring in the system. Unfortunately, in reality this is not possible due to several effects.

To build up good thermal stratification means that the inlet flow into the tank enters the tank at the height where the tank temperature is equal to the inlet temperature. To reach this goal, in principle two possibilities are possible:

• A hydraulic device, a so-called stratifier, is used to achieve that the flow enters the tank at the level according where the inlet temperature fits to the tank temperature.

Such devices can be different kinds of self acting stratifiers mounted inside or beside the tank. Alternatively, the flow is controlled by switching valves in combination with several pipes connected at the tank in different heights.

• According to few fixed inlets where the temperature in the tank is measured, the inlet temperature is controlled in a way that it fits to the tank temperature. For example, the collector forward flow temperature in combination with a “matched flow” control strategy can be controlled in such a way.

In simulation studies (Andersen et.al. 2006; 2007) of medium sized solar combisystems with 20 m² collector area, 1000 liter solar tank and different total heat loads between about 5,500 and 18,000 kWh per year the influence of theoretical perfect stratifiers on the energy savings was investigated.

For the case of inlet temperatures, due to variable space heating return temperatures, simulations have shown that a perfect inlet stratifier can improve the energy savings by 2 to 6 %. For the case of variable solar forward temperatures due to changing irradiation a perfect inlet stratifier can improve the energy savings by 5 to 8 %. When using perfect inlet stratifiers for both cases in parallel, the calculations showed increased energy savings between 7 and 14 %. The simulations also showed, the higher the space heating load is, the higher the advantage of the stratifier.

If good thermal stratification is built up, it is also important to keep the thermal stratification as good as possible over time.

Thermal stratification can easily be destroyed by turbulences due to badly designed inlets. The inlet flow direction, first of all, should be horizontal and not vertical.

Further, the inlet velocity should be very low; less than 0.03 m/s are recommended (Furbo 2004).

Beside a good insulation quality of the tank and no thermal bridges, especially in the hot, top part of the tank, low vertical heat conductivity is essential to keep the thermal stratification. Water itself has a relatively low heat conductivity (0.6 W/mK), but steel has a relatively high heat conductivity (50 W/mK). Therefore, it should be avoided to use steel inside the tank for devices like internal vertical pipes or frames to fix other components. Plastic material with a heat conductivity of only 0.35 W/mK (cross-linked Polyethylene, also called PEX), which can withstand temperatures up to 100°C is much better for such purpose.

As part of a very detailed simulation study on pipe connections at a tank (see chapter 4.3, page 57) (Thür et.al. 2005), it was also investigated how big is the influence of internal pipes (going from the bottom to the desired height) on the thermal stratification in the tank and therefore on the auxiliary demand. The major conclusions were that PEX pipes with 2 mm wall thickness and 20 mm outer diameter, which are used for three internal pipes in the tank, lead to 2.2 % higher auxiliary demand compared to ideal internal pipes. If the wall thickness of the PEX pipes is increased from 2 mm to 8 mm the auxiliary demand is only 1.2 % higher compared to ideal internal pipes (see Fig. 4–12, page 67). The difference of 1 %-point

based on a typical annual heat load of about 20,000 kWh is equivalent to 200 kWh, which again is equivalent to the total annual solar gain of almost 1 m² collector area.

2.5.2 Low Temperature – High Power Control Concept

Good utilization of heat also means that in the best case, heat should be generated exactly at the right temperature and at the time when it is needed. In combination with a fast and powerful condensing natural gas boiler this can be realized. The principle is to use a sufficiently large auxiliary volume as part of the solar tank, which can be used by the boiler to operate at good operating conditions for all conditions during space heating. This means that the boiler can operate at low temperature and at least at the minimum power that can be reached by modulation and is not forced to operate below this minimum power.

High power and high temperature for hot water preparation are produced exactly when hot water is tapped. Therefore, it is not necessary to store heat at high temperature for long time without being used and just generating heat losses.

Summarizing up all the discussed details in this chapter, the goal is to design a system concept, which ensures high efficiency during heat production (solar collector and boiler), good utilization of heat that is available in the heat storage, and finally low heat losses. The next chapter will describe one possibility how such a new developed system concept, based on the specific boundary conditions described in chapter 2.1 (page 12), can look like.