3.5 Pumps
3.5.1 Pump curve
Pump performance is usually described with two characteristics, a head and a flow rate, the head as a function of the flow rate. This is called a pump curve. For a centrifugal pump the head normally decreases when the flow rate increases as shown in Picture 6. A point where both curves, the pump curve and a pipe resistance curve, meet, is called a duty point. In that single point of the pump curve, the pump operates.
Picture 5. The blue curve at above picture describes the pump performance, the head as the function of the flow rate (Grundfos 2019).
The duty point can be changed, when either pipe resistance or pumps rotational speed change. Also, an impeller diameter effects to pump performance. When pipe resistance increases, the resistance curve becomes steeper, as shown in Picture 7. Increasing the pipe resistance results from changes in the pipeline. For example, if a valve is shut in the pipeline, the friction losses lead to higher pressure drop. Other possibilities are longer pipeline or smaller pipe diameters. At the same way, if the control valve is opened, flow rate trough the pump increases due the smaller pipe resistance.
Picture 6. The resistance curve steepens, when pressure lost increases (Grundfos 2019).
Controlling the flow rate can be used by changing the rotational speed of the pump. This is made with frequency converter. The pump’s motor may have an integrated frequency converter, or it can be a separate device. Whit variable speed drive the resistance curve stays still, but the pump curve lowers or higher depend on the change of speed. If the frequency is lowered, also the pumps curve lowers. This is how a lower flow rate can be driven without increasing the pressure drop, as in Picture 8.
Picture 7. The pump can be driven with a frequency converter (Grundfos 2019).
Resistance curves in Picture 6 and Picture 8 are same. In Picture 6 the rotational speed is 2936 rpm and the frequency is 50 Hz. When lowering the frequency to 38,7 Hz, rotational speed is 2292 rpm and the flow rate decreases to 20 m3/h. The head of the pump is 11,75 m. If the same flow rate is driven without variable speed, but shutting a control valve, the head is higher 29,2 m. A useful power of the pump, as Johann F. Gülich (Gülich 2008, 46) describes it, is the ideal power needed to rise pressure of the fluid.
𝑃𝑃u= 𝜌𝜌𝜌𝜌H𝑞𝑞v (9)
Pu useful pump power [W]
g acceleration of gravity [m/s2]
H pump head [m]
qv flow rate [m3/s]
The useful pump power is calculated at the duty point in Picture 6 with equation 9. The flow rate is 25,6 m3/h, which equals to 0,0071 m3/s. The head is 19,18 m and density 998,2 kg/m3.
𝑃𝑃u= 998,2 kg
m3∙9,82m
s2∙19,18 m∙0,0071m3
s = 1,336 kW
The useful pump power doesn’t take account a pump’s efficiency. The efficiency can be read from Picture 6.
𝑃𝑃1 = 𝑃𝑃u
𝜂𝜂𝑝𝑝 (10)
P1 pump power [W]
ηp pumps efficiency [-]
The pump efficiency at duty point 1, Picture 6, is 69 %. The pump power is calculated with equation 10.
𝑃𝑃1 =1,336 kW
0,69 = 1,936 kW
The pump power is calculated in duty point 1, 2 and 3 (Picture 6, Picture 7, Picture 8) as presented in Table 6.
Table 6. The rotational speed is same at duty points 1 and 2.
The useful pump power is highest at duty point one, but it has the highest flow rate. When flow rate is reduced by closing the valve, head of the pumps increases. More efficient way of reducing flow rate is to lower the rotational speed as at duty point three. The flow rate is same as at duty point two, but head is much lower, only 11,75 m. The duty point three has better efficiency and lower pump power.
The pump performance in variable speed drive can also be approximated by affinity laws.
The affinity laws apply when there is now changes in piping or any other conditions. This means, that the resistance curve stays similar. (Skovgaard & Nielsen 2004, 109.)
𝑞𝑞𝑣𝑣,n
𝑃𝑃1,n 𝑃𝑃1,x = (𝑛𝑛n
𝑛𝑛x)3 (13)
P1,n pump power in nominal duty point [W]
P1,x pump power in duty point x [W]
The head and the pump power in duty point three can be calculated with equations 11, 12 and 13. The head with flow rate 20,1 m3/h is calculated based on the duty point one.
𝐻𝐻1
Calculated values for head and power in duty point three differ from values in Table 6.
The affinity laws are approximations, so they don’t give the exact values. The head read from the pump curve in duty point three is 11,75 m and calculate head 12,29 m. The pump power calculated with equation 10 is 0,922 kW and with the affinity law the power is 0,994 kW.
4 LOW TEMPERATURE DISTRICT HEATING
Heating and cooling of buildings are one of the major energy sectors worldwide.
Approximately one third of the energy consumption comes from heating and cooling.
Low temperature district heating has been discussed and studied for a while. An aim to achieve 100% renewable energy system inquires changes in district heating production and therefore in distribution. By lowering the supply and return temperature new sources of energy could be used for heating. (Lund et al. 2017, 5-6).
Lowering supply temperature is part of a concept “4th generation district heating” or 4DH.
This concept tries to emphase and concur the future challenges of district heating. Aim to use more renewable energy sources and reduction of heat demand due to more energy efficient buildings affect to district heating production and distribution. Production has been quite traditional with large central heating plants and conventional fuels. Current supply temperatures don’t make achievable to use e.g. heat pumps and surplus heat for production. Reduce temperature would also improve efficiency in production plant and lower heat losses. (Lund et al. 2017, 6).
4.1 District heating competitiveness in Finland
Regarding EU’s aim for year 2021, every new building should be almost zero energy buildings. This means that over all energy balance is almost zero. This might affect to district heating demand because more energy supplied from outside to buildings, more energy need to be produced e.g. with solar panels. While all new heating methods, such as geothermal heat pumps, have been developed, the district heating needs to develop to meet the new standards and customer wishes. Valtion teknillinen tutkimuskeskus VTT (Finnish government technical research center) has published a research “Tulevaisuuden kaukolämpöasuinalueen energiaratkaisut” with some energy companies, to investigate how to include district heating for buildings with low energy consumption. (Klobut et al.
2014 7).