© ZSW

Using algo­rithms to oper­ate heat pumps more effi­cient­ly and in a more grid-friend­ly manner

Heat pumps pro­vide envi­ron­men­tal­ly friend­ly heat­ing and hot water. They are there­fore a key tech­nol­o­gy for a cli­mate-neu­tral build­ing stock. How­ev­er, an increas­ing num­ber of these future-proof heat­ing sys­tems could place an exces­sive bur­den on the elec­tric­i­ty dis­tri­b­u­tion net­works. The Cen­tre for Solar Ener­gy and Hydro­gen Research Baden-Würt­tem­berg (ZSW) has now devel­oped algo­rithms that reduce peak loads. The researchers test­ed the new method in Swe­den, where heat pumps are already wide­spread and win­ters are par­tic­u­lar­ly cold. The result: The algo­rithms help to oper­ate heat pumps effi­cient­ly and in a way that serves the grid. The load on the trans­form­ers in the dis­tri­b­u­tion grid decreased by ten percent.

Fore­casts pre­dict that the share of heat pumps in the Ger­man heat­ing mix will rise sharply in the com­ing years. For low-volt­age dis­tri­b­u­tion net­works in res­i­den­tial areas, this could become a bur­den with­out read­just­ment. Because: When it gets cold out­side, all heat pumps deliv­er a high heat­ing out­put at the same time — espe­cial­ly in the morn­ing and at night. The demand for elec­tric­i­ty in the dis­tri­b­u­tion grid is increas­ing accord­ing­ly. High­er load peaks then occur in the grids and at the trans­form­ers that con­vert the volt­age in the upstream medi­um-volt­age grid to the volt­age in the dis­tri­b­u­tion grid. That could over­load them.

The new method is one of many that the ZSW has devel­oped for grid oper­a­tors, man­u­fac­tur­ers and users. The aim is to be able to use large con­sumers such as heat pumps and e‑charging sta­tions even in exist­ing grids through intel­li­gent oper­a­tion, if pos­si­ble with­out any notice­able restriction.

Heat pumps do not have to bur­den dis­tri­b­u­tion grids

The researchers have there­fore devel­oped algo­rithms to reduce the simul­tane­ity of heat pump loads in a grid area. “The chal­lenge is to pro­vide a warm house for every­one in the ear­ly morn­ing and evening with­out all the heat pumps start­ing up at the same time — even on days when the out­side tem­per­a­ture is minus ten degrees Cel­sius,” explains Dr. Jann Binder from ZSW. “To do this, we have devel­oped a pre­dic­tive heat pump oper­a­tion that uses a fore­cast of heat demand.”

In the event of a fore­see­able mains load, the heat pump switch­es on ear­li­er and runs longer, but at a low­er out­put. The process uses the heat capac­i­ty of the house as a stor­age medi­um and thus relieves the load on the grid. The researchers use this in a well-dosed man­ner so as not to increase the heat loss sig­nif­i­cant­ly and to keep the result­ing tem­per­a­ture devi­a­tion from the set­point with­in limits.

There were two approach­es to choose from: a cen­tralised approach, in which house­hold heat pumps are incen­tivised to oper­ate in a dis­trib­uted man­ner by a cen­tral office via vir­tu­al ener­gy pric­ing, and a decen­tralised approach, in which heat pumps sim­ply respond to local­ly sensed tem­per­a­ture fluc­tu­a­tions and reduc­tions in grid volt­age, with no con­nec­tion to a cen­tral office. The cen­tralised approach achieves the required grid relief of ten per­cent with three per­cent less addi­tion­al expen­di­ture on heat­ing ener­gy than the decen­tralised approach, as it can avoid the need for “pre-heat­ing” and the simul­tane­ity of heat pump oper­a­tion more pre­cise­ly. How­ev­er, it requires a large num­ber of cal­cu­la­tions to deter­mine the indi­vid­ual sched­ules and thus more com­mu­ni­ca­tion effort between all heat pumps and the con­trol centre.

Ten per­cent less peak loads at the transformers

The result for the sim­pler decen­tral­ized approach: With the ten per­cent reduc­tion of the trans­former load at peak times, the spread of the indoor tem­per­a­ture changed only min­i­mal­ly; from 20 to 22 degrees Cel­sius to 19.2 to 22.2 degrees. If one addi­tion­al­ly uses a fore­cast of the trend of the out­side tem­per­a­ture, the low­est tem­per­a­ture is even lim­it­ed to 19.4 degrees. If the same reduc­tion in trans­former load were to be achieved by lin­ear reduc­tion in heat pump out­put alone, the min­i­mum indoor tem­per­a­ture would be 17 degrees, which is three degrees less, not just 0.6 degrees less.

When devel­op­ing the decen­tral­ized approach, the ZSW paid atten­tion to a sim­ple design. “The algo­rithm does not need an exter­nal com­mu­ni­ca­tion link for remote con­trol of the heat pumps,” says Binder. “Local­ly mea­sured line volt­age is used as the source of infor­ma­tion.” If the volt­age falls below a lim­it val­ue, this is an indi­ca­tion that the mains load is too high. As a result, the algo­rithm kicks in and mod­u­lates the heat pump out­put. Com­pared to a cen­tral con­trol of heat pumps with­out com­plex bidi­rec­tion­al com­mu­ni­ca­tion, a decen­tral­ized algo­rithm can use the abil­i­ty of the house to store heat in an indi­vid­ual and well-dosed way. This reduces the result­ing tem­per­a­ture drop com­pared to that which would occur if heat pumps were switched off cen­tral­ly in the event of grid bottlenecks.

Test of the pro­ce­dure in Sweden 

In Swe­den, the influ­ence of heat pumps on the load on the elec­tric­i­ty grid and their grid-serv­ing oper­a­tion can already be stud­ied very well today. The Scan­di­na­vian coun­try has a high car­bon tax, so the use of heat pumps is already widespread.

The ZSW researchers chose the Ram­sjö test site near Stock­holm. Here, the hous­es are main­ly heat­ed with heat pumps. An ide­al test area: In win­ter, the trans­form­ers were heav­i­ly loaded dur­ing par­tic­u­lar­ly cold weather.

Why heat pumps are climate-friendly 

Heat pumps are in vogue. Around 53 per­cent of all new build­ings were equipped with the tech­nol­o­gy last year. The pro­por­tion is low­er for the replace­ment of heat­ing sys­tems in exist­ing build­ings. How­ev­er, their use will also increase sig­nif­i­cant­ly in ren­o­vat­ed res­i­den­tial build­ings in the future. The rea­son: on the way to cli­mate neu­tral­i­ty, the heat­ing sec­tor must say good­bye to nat­ur­al gas and oil.

Heat pumps help with this con­ver­sion. The devices obtain most of their ener­gy from their direct envi­ron­ment — the air or the ground. The heat from the envi­ron­ment is renew­able and avail­able in prac­ti­cal­ly unlim­it­ed quan­ti­ties. To raise the tem­per­a­ture to the required lev­el, heat pumps need elec­tric­i­ty, which increas­ing­ly comes from wind ener­gy and pho­to­volta­ic plants. This makes the tech­nol­o­gy more cli­mate-friend­ly year after year. On aver­age, three to five kilo­watt hours of heat can be gen­er­at­ed from one kilo­watt hour of elec­tric­i­ty per year, depend­ing on the oper­at­ing con­di­tions and the tech­nol­o­gy used in the heat pumps.

The research project was part of the NEMoGrid project fund­ed by the Ger­man Fed­er­al Min­istry for Eco­nom­ic Affairs and Ener­gy (BMWi) (fund­ing code 0350016A). The term was just over three years and end­ed on 31 Decem­ber 2020.