Design of solar combi systems

It is not obvious how solar combi systems should be designed. In section 2.2 examples of solar combi systems on the European market are shown. In Figure 2.1 the distribution of volumes charged and discharged during operation in the hot water heat storage of a solar combi system are schematically illustrated.

Figure 2.1 Schematical illustration of the storage tank of a solar combi system.

The domestic cold water inlet and domestic hot water outlet are situated at the bottom and the top of the domestic hot water tank, respectively (ref. Figure 2.1). The outlet for domestic hot water can be in different additional levels, also below the auxiliary heated volume. In this way domestic hot water can be discharged with different temperatures. Domestic hot water can also be discharged from the inner tank in a tank-in-tank, with external heat exchangers or immersed heat exchanger spirals as shown in Figure 2.2.

Volume for space heating Volume for

solar collector Volume for auxiliary energy

to auxiliary boiler

from auxiliary boiler domestic hot water

domestic cold water from solar collector

to space heating system

from space heating system

to solar collector

Volume for heating domestic hot water

Figure 2.2 Options for domestic hot water discharge. Left: Domestic hot water tank-in-tank. Middle: Sidearm and plate heat exchanger. Right: Immersed heat exchanger spiral.

The outlet for space heating is usually above the lower level of the auxiliary heated volume. Charge and discharge of the storage tank can be done with external heat exchangers or immersed heat exchanger spirals or with direct inlets and outlet.

The storage tank is heated by a solar collector and by an auxiliary energy supply system. It also can be a pre-heating system with a storage tank only heated by a solar collector. The system must be able to supply both domestic hot water and space heating in an effective way. Domestic hot water supply usually requires temperatures higher than 45 ºC, whereas space heating most of the time requires lower temperatures, but the temperature is of course depending on the size of the heating system and the heating demand of the house. Dimensioning flow temperatures and return temperatures of 60ºC / 40ºC or 70ºC / 50ºC with volume flow rates of about 300 l/hr are usually used for traditional radiator heating systems. In floor heating systems, the dimensioning flow temperature is in the range of 35ºC - 40ºC with dimensioning return temperature in the range 30ºC - 35ºC and the volume flow rate is in the range of 1000 l/hr.

The solar collector loop is usually operated with a propylene glycol-water mixture (anti freeze liquid) but can also be operated with water. The latter requires a drain back system where the solar collector is emptied when not in operation. The volume flow rate in the solar collector loop is driven by a pump and is in the range from 0.15 l/min/m² – 1.2 l/min/m². The low volume flow rate results in high temperature differences between the inlet and the outlet of the solar collector and the high volume flow rate results in low temperature differences.

The energy from the solar collector operated with high volume flow rate in the solar collector loop is usually transferred to the storage tank through a heat exchanger spiral immersed at the bottom of the storage tank (ref. left in Figure 2.3) or through an external heat exchanger with pipe connections to the tank (ref. right in Figure 2.3). In the latter approach, solar energy on the secondary side of the heat exchanger is transferred to the storage tank by a pump.

domestic hot water domestic cold water

domestic hot water

domestic cold water domestic

cold water domestic hot water

Figure 2.3 Options for transferring energy from the solar collector loop to the heat storage tank with high volume flow rate in the solar collector loop.

Low volume flow rates are used in solar heating systems based on storage tank designs with emphasis on enhancing thermal stratification in the heat storage tank.

High volume flow rates are, due to the lower temperature level of the collector outlet, less suitable for this approach.

Low volume flow rate operation in connection with an immersed heat exchanger does normally not provide any thermal advantage. To benefit from a low volume flow rate in the solar collector loop, the solar heat must be transferred to the tank in a level where the temperature of the tank water is close to the temperature produced by the solar collector. Thermal stratification can be achieved, for example by using inlet stratification devices at all inlets to the storage tank. Figure 2.4 show examples of inlet stratification design options in the solar collector loop.

from solar collector

to solar collector

from solar collector to solar collector

Figure 2.4 Stratification design options schematically illustrated with stratification pipes (fabric inlet stratifiers), mantle tank, external heat exchanger and heat exchanger spirals. Top left: External heat exchanger with either thermosyphoning or pump driven volume flow rate on the secondary side of the heat exchanger. Top middle: Internal heat exchanger with thermosyphoning volume flow rate in the heat storage tank. Top right:

Direct inlet (e.g. drain back system). Bottom left: Mantle tank. Bottom middle: External heat exchanger with multiple inlets to the tanks with either thermosyphoning or pump driven volume flow rate on the secondary side of the heat exchanger. Bottom right: Two heat exchanger spirals with inlet to the upper, the lower or both heat exchanger spirals.

Thermal stratification in the storage tank is extremely important in order to achieve high thermal performance of a solar heating system. High temperatures in the top of the storage tank and low temperatures in the bottom of the storage tank lead to the best operation conditions for any solar heating system. High temperatures in the top of the storage tank established by the energy from the solar collector, reduce the use of auxiliary energy. Low temperatures in the bottom of the storage tank improve the operation conditions for the solar collector. The solar collector can easily transfer energy to the storage tank and can be in operation for a longer period leading to a better utilization of the solar collector.

Solar energy can be stored in a domestic hot water tank where domestic hot water is lead directly from the tank to the consumer or in a space heating tank with a heat exchanger between the tank water and the domestic hot water. When storing energy in the domestic water, the retention time of the water in the tank must be considered.

Long retention time in the tank in combination with a temperature level in the range 30ºC - 45ºC can create an optimal environment for bacteria growth, e.g. legionella in the tank. Investigations of legionella in storage tanks have showed that legionella

from solar collector to solar collector

Inlet stratifier

from solar collector

to solar collector Inlet stratifier

from solar collector

to solar collector

from solar collector to solar collector from solar

collector

to solar collector Inlet stratifier

from solar collector

to solar collector

bacteria can not multiply at temperatures below 20 ºC and above 50 ºC and not survive temperatures above 60ºC (Cabeza 2005).

Most solar heating systems are depending on an auxiliary energy supply system. The auxiliary energy supply system can be a gas or an oil boiler, an electrical heater or based on wood or pellet burners. Most auxiliary energy supply systems are connected to the solar heating system as separate units but also solar heating systems with integrated auxiliary energy supply systems exist, for instance solar heating systems with integrated gas boilers (e.g. Solvis). For auxiliary energy supply systems that utilize the energy from the exhaust gas by condensing, it is extremely important that the return temperature to the auxiliary energy supply system is sufficiently low to enable condensing of the exhaust gas. These boilers are more expensive than non-condensing boilers and therefore a bad choice if the non-condensing feature is not utilized.

Only one control system for controlling the solar collector loop, the auxiliary energy system and the space heating loop should be used. When two independent control systems are used, there is constantly the risk that the auxiliary energy supply system heats up the auxiliary volume during periods when the heating could be easily managed by the solar collector. This results in higher auxiliary energy consumption than necessary and worse operation conditions for the solar collector.

High heat losses from the storage tank reduce the thermal performance of the solar heating system, because the heat losses are partly covered by the auxiliary energy supply system. Heat losses can not be avoided, but kept at a minimum by insulating all parts of the storage tank carefully and in such a way that convection of air between the tank and the insulation material can not take place. Heat losses can be further reduced by keeping all pipe connections in the lower and colder part of the storage tank and by reducing the set point temperature of the auxiliary energy system and the volume for auxiliary energy as much as possible.

It is most common to design small solar combi systems with short term storage with capacity for only a few days, but also large solar combi systems with long term storage are used. In these systems heat collected during summer is stored and used during the heating season.